diff --git a/pipeline/download/test_transit_network.py b/pipeline/download/test_transit_network.py
new file mode 100644
index 0000000..c50e654
--- /dev/null
+++ b/pipeline/download/test_transit_network.py
@@ -0,0 +1,79 @@
+"""Tests for transit_network GTFS processing."""
+
+import zipfile
+from pathlib import Path
+
+import pytest
+
+from pipeline.download.transit_network import convert_high_freq_to_frequency_based
+
+
+def _write_gtfs(path: Path, *, stop_times: str) -> None:
+    """Write a minimal GTFS zip with one metro route and several trips."""
+    routes = "route_id,route_type\nR1,1\n"
+    trips = "trip_id,route_id,direction_id,service_id\n" + "".join(
+        f"T{i},R1,0,S1\n" for i in range(1, 7)
+    )
+    with zipfile.ZipFile(path, "w") as z:
+        z.writestr("routes.txt", routes)
+        z.writestr("trips.txt", trips)
+        z.writestr("stop_times.txt", stop_times)
+
+
+def _one_based_stop_times() -> str:
+    """Six trips, 1-based stop_sequence (1,2,...), 5-minute headway."""
+    header = "trip_id,stop_sequence,departure_time,stop_id\n"
+    rows = []
+    # First departures 06:00, 06:05, ... (300s = 5 min headway, well under 15 min)
+    for i in range(6):
+        trip = f"T{i + 1}"
+        first_dep = 6 * 3600 + i * 300
+        h, m = divmod(first_dep, 3600)
+        m, s = divmod(m, 60)
+        # First stop has stop_sequence 1 (NOT 0); second stop sequence 2.
+        rows.append(f"{trip},1,{h:02d}:{m:02d}:{s:02d},STOP_A\n")
+        later = first_dep + 120
+        h2, m2 = divmod(later, 3600)
+        m2, s2 = divmod(m2, 60)
+        rows.append(f"{trip},2,{h2:02d}:{m2:02d}:{s2:02d},STOP_B\n")
+    return header + "".join(rows)
+
+
+def test_one_based_stop_sequence_is_converted(tmp_path: Path) -> None:
+    """First stop selection must use the minimum stop_sequence, not literal "0".
+
+    With 1-based stop_sequence the old code (keyed on stop_sequence == "0") found
+    zero first stops and produced an empty frequencies.txt. The fix selects the
+    minimum stop_sequence per trip, so the high-frequency group is converted.
+    """
+    src = tmp_path / "in.zip"
+    dst = tmp_path / "out.zip"
+    _write_gtfs(src, stop_times=_one_based_stop_times())
+
+    convert_high_freq_to_frequency_based(src, dst)
+
+    with zipfile.ZipFile(dst, "r") as z:
+        freq = z.read("frequencies.txt").decode("utf-8")
+
+    freq_rows = [r for r in freq.splitlines()[1:] if r.strip()]
+    # The single high-frequency group must produce exactly one frequency entry.
+    assert len(freq_rows) == 1, freq
+    trip_id, start_time, end_time, headway_secs, _exact = freq_rows[0].split(",")
+    # Template trip is the earliest departure (T1 at 06:00) starting at first stop.
+    assert start_time == "06:00:00"
+    # Median headway of 300s rounds to a 300s headway entry.
+    assert headway_secs == "300"
+
+
+def test_raises_when_no_first_stops_found(tmp_path: Path) -> None:
+    """A non-empty target trip set with unparseable stop_sequence is loud, not silent."""
+    src = tmp_path / "in.zip"
+    dst = tmp_path / "out.zip"
+    bad = (
+        "trip_id,stop_sequence,departure_time,stop_id\n"
+        "T1,not_a_number,06:00:00,STOP_A\n"
+    )
+    _write_gtfs(src, stop_times=bad)
+
+    with pytest.raises(RuntimeError, match="no first stops"):
+        convert_high_freq_to_frequency_based(src, dst)
diff --git a/pipeline/transform/postcode_boundaries/fragments_cache.py b/pipeline/transform/postcode_boundaries/fragments_cache.py
new file mode 100644
index 0000000..5fa5b4d
--- /dev/null
+++ b/pipeline/transform/postcode_boundaries/fragments_cache.py
@@ -0,0 +1,79 @@
+"""Persist Phase-3 output (the per-postcode fragments) so a crash in the later
+merge/write phases can resume in seconds instead of re-running the ~10-hour OA
+loop.
+
+Phase 3 turns OA boundaries + INSPIRE parcels + UPRN points into ~1.8M
+``(postcode, geometry)`` fragments held only in memory. Everything after it
+(merge, simplify, GeoJSON write) is cheap but failure-prone -- a single
+degenerate postcode used to abort the whole run *after* those 10 hours. Caching
+the fragments to disk decouples the expensive computation from the fragile
+output stage.
+
+Fragments are stored as one parquet file with two columns: ``postcode``
+(string) and ``wkb`` (binary Shapely WKB). Writes are atomic (temp file +
+``os.replace``) so an interrupted write never leaves a cache that passes the
+freshness check. The cache is validated against its upstream inputs by mtime: if
+any input is newer than the cache it is treated as stale and ignored, mirroring
+make's own freshness logic.
+"""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+import numpy as np
+import polars as pl
+import shapely
+from shapely.geometry.base import BaseGeometry
+
+Fragment = tuple[str, BaseGeometry]
+
+
+def _tmp_path(cache_path: Path) -> Path:
+    return cache_path.parent / (cache_path.name + ".tmp")
+
+
+def fragments_cache_is_fresh(
+    cache_path: Path, inputs: list[Path | None]
+) -> bool:
+    """True if ``cache_path`` exists and is newer than every input that exists.
+
+    A missing input is ignored (it cannot have changed the cache); a ``None``
+    input is skipped. Any existing input newer than the cache marks it stale.
+    """
+    if not cache_path.exists():
+        return False
+    cache_mtime = cache_path.stat().st_mtime
+    for inp in inputs:
+        if inp is None:
+            continue
+        path = Path(inp)
+        if path.exists() and path.stat().st_mtime > cache_mtime:
+            return False
+    return True
+
+
+def save_fragments(cache_path: Path, fragments: list[Fragment]) -> None:
+    """Atomically write ``(postcode, geometry)`` fragments to a parquet cache."""
+    postcodes = [pc for pc, _ in fragments]
+    geoms = np.array([geom for _, geom in fragments], dtype=object)
+    wkb = shapely.to_wkb(geoms)
+
+    frame = pl.DataFrame(
+        {"postcode": postcodes, "wkb": list(wkb)},
+        schema={"postcode": pl.Utf8, "wkb": pl.Binary},
+    )
+
+    cache_path.parent.mkdir(parents=True, exist_ok=True)
+    tmp = _tmp_path(cache_path)
+    frame.write_parquet(tmp, compression="zstd")
+    os.replace(tmp, cache_path)
+
+
+def load_fragments(cache_path: Path) -> list[Fragment]:
+    """Read fragments written by :func:`save_fragments` back into memory."""
+    frame = pl.read_parquet(cache_path)
+    postcodes = frame["postcode"].to_list()
+    geoms = shapely.from_wkb(frame["wkb"].to_list())
+    return list(zip(postcodes, geoms))
diff --git a/pipeline/transform/price_estimation/test_shrinkage.py b/pipeline/transform/price_estimation/test_shrinkage.py
new file mode 100644
index 0000000..06ab01f
--- /dev/null
+++ b/pipeline/transform/price_estimation/test_shrinkage.py
@@ -0,0 +1,99 @@
+"""Regression tests for common-base-year re-anchoring before blending.
+
+Each repeat-sales index dict is anchored to log-index 0 at its OWN earliest
+year. shrink_dicts / blend_dicts combine dicts key-by-key, so dicts anchored to
+different base years must be re-anchored to a single common base first, or the
+blend averages level-incompatible numbers (fix5-index-base-year).
+"""
+
+from pipeline.transform.price_estimation.shrinkage import (
+    blend_dicts,
+    reanchor_dict,
+    reanchor_dicts,
+    shrink_dicts,
+)
+
+
+def test_reanchor_is_pure_constant_shift_preserving_differences():
+    """Re-anchoring only shifts the origin; year-to-year moves are unchanged."""
+    # Anchored at its own earliest year 2008.
+    idx = {2008: 0.0, 2009: 0.10, 2010: 0.25, 2011: 0.40}
+
+    reanchored = reanchor_dict(idx, 1996)
+    # 1996 is before this dict's history -> back-fill earliest value (0.0),
+    # so the shift is 0 and the dict is unchanged.
+    assert reanchored[2008] == 0.0
+
+    # Same shape, different exact-hit base year: anchoring at 2010 subtracts 0.25.
+    reanchored_2010 = reanchor_dict(idx, 2010)
+    assert reanchored_2010[2010] == 0.0
+    # All within-dict differences are preserved under the constant shift.
+    years = sorted(idx)
+    for a, b in zip(years, years[1:]):
+        assert abs((reanchored_2010[b] - reanchored_2010[a]) - (idx[b] - idx[a])) < 1e-12
+
+
+def test_blend_different_base_years_needs_reanchoring():
+    """Blending two dicts on different bases is biased unless re-anchored first.
+
+    Both cells observe the common base year 1996 but were anchored to DIFFERENT
+    origins (sectorA at 1996, sectorB at 2008, as solve_robust_index would do for
+    cells whose pair history starts at different years). They describe the SAME
+    true trajectory measured from 1996, so a 50/50 blend should reproduce that
+    common level. Pre-fix, blend_dicts mixes sectorB's 2008-relative numbers with
+    sectorA's 1996-relative numbers, level-shifting the smoothed result.
+    """
+    base_year = 1996
+
+    # True log-levels relative to 1996 (identical trajectory for both cells).
+    truth = {1996: 0.0, 2008: 0.80, 2012: 1.00}
+
+    # sectorA: anchored at 1996 (its earliest year) -> equals truth.
+    sector_a = dict(truth)
+    # sectorB: same trajectory but anchored at 2008 (subtract truth[2008] from
+    # every year), exactly how solve_robust_index would express a cell whose
+    # earliest year happened to be picked as 2008.
+    shift_b = truth[2008]
+    sector_b = {y: v - shift_b for y, v in truth.items()}
+
+    # --- Pre-fix behaviour: blend the raw dicts directly. ---
+    raw_blend = blend_dicts(sector_a, [sector_b], 0.5, [0.5])
+    # Every year is pulled by half of shift_b (0.4) away from the truth.
+    assert abs(raw_blend[2012] - truth[2012]) > 0.3
+    assert abs(raw_blend[1996] - truth[1996]) > 0.3
+
+    # --- Post-fix behaviour: re-anchor to the common base, THEN blend. ---
+    reanchored = reanchor_dicts({"A": sector_a, "B": sector_b}, base_year)
+    fixed_blend = blend_dicts(reanchored["A"], [reanchored["B"]], 0.5, [0.5])
+    # Both cells now read 0 at 1996 and the true level at every shared year.
+    for y in truth:
+        assert abs(fixed_blend[y] - truth[y]) < 1e-9
+
+
+def test_shrink_dicts_after_reanchoring_is_consistent():
+    """Shrinking a cell toward its parent must use a common origin."""
+    base_year = 2000
+    # Parent (national) anchored at 2000.
+    parent = {2000: 0.0, 2010: 0.50, 2020: 1.20}
+    # Sector tracking the parent exactly but anchored at 2010 (subtract 0.50 from
+    # every year), as solve_robust_index would express a cell whose earliest year
+    # is later. It still observes the 2000 base year (value -0.50).
+    sector = {2000: -0.50, 2010: 0.0, 2020: 0.70}
+    n = 0  # no own data weight -> result should equal parent after anchoring
+
+    reanchored_sector = reanchor_dict(sector, base_year)
+    # Exact hit on 2000 subtracts -0.50, putting the sector back on the parent's
+    # origin: 0.0 at 2000, 0.50 at 2010, 1.20 at 2020.
+    shrunk = shrink_dicts(reanchored_sector, parent, n)
+    assert abs(shrunk[2000] - 0.0) < 1e-9
+    assert abs(shrunk[2010] - 0.50) < 1e-9
+    assert abs(shrunk[2020] - 1.20) < 1e-9
+
+
+def test_reanchor_exact_hit_shifts_all_years():
+    """When the base year is present, subtract its value from every year."""
+    idx = {1996: 0.0, 2005: 0.30, 2015: 0.90}
+    reanchored = reanchor_dict(idx, 2005)
+    assert reanchored[2005] == 0.0
+    assert abs(reanchored[1996] - (-0.30)) < 1e-12
+    assert abs(reanchored[2015] - 0.60) < 1e-12
diff --git a/pipeline/utils/test_postcode_mapping.py b/pipeline/utils/test_postcode_mapping.py
new file mode 100644
index 0000000..3ed0c0f
--- /dev/null
+++ b/pipeline/utils/test_postcode_mapping.py
@@ -0,0 +1,38 @@
+import polars as pl
+
+from pipeline.utils.postcode_mapping import (
+    MAX_REMAP_DISTANCE_M,
+    build_postcode_mapping,
+)
+
+
+def test_remap_drops_terminated_postcodes_beyond_distance_cap(tmp_path):
+    # One active England postcode at the origin.
+    # One terminated postcode 100m away (legitimate adjacent remap -> mapped).
+    # One terminated postcode 5km away (gross misattribution -> dropped).
+    arcgis = pl.DataFrame(
+        {
+            "pcds": ["AB1 1AA", "AB1 1AB", "ZZ9 9ZZ"],
+            "ctry25cd": ["E92000001", "E92000001", "E92000001"],
+            "doterm": [None, "202001", "202001"],
+            "east1m": [500000.0, 500100.0, 505000.0],
+            "north1m": [200000.0, 200000.0, 200000.0],
+        }
+    )
+    arcgis_path = tmp_path / "arcgis.parquet"
+    arcgis.write_parquet(arcgis_path)
+
+    mapping = build_postcode_mapping(arcgis_path)
+
+    # The nearby terminated postcode is remapped onto the active one.
+    assert mapping.filter(pl.col("old_postcode") == "AB1 1AB")[
+        "new_postcode"
+    ].to_list() == ["AB1 1AA"]
+
+    # The far (5km > 1km cap) terminated postcode must NOT appear in the mapping.
+    assert "ZZ9 9ZZ" not in mapping["old_postcode"].to_list()
+    assert mapping.height == 1
+
+
+def test_max_remap_distance_constant_is_one_kilometre():
+    assert MAX_REMAP_DISTANCE_M == 1000.0
diff --git a/probe_bin b/probe_bin
new file mode 100755
index 0000000..e89bbf9
Binary files /dev/null and b/probe_bin differ