diff --git a/c6_cohort.py b/c6_cohort.py
new file mode 100644
index 000000000..25c18c5cd
--- /dev/null
+++ b/c6_cohort.py
@@ -0,0 +1,41 @@
+"""Which 21.0.1-corpus certs are in #1606's C6 half-charge cohort?
+Cohort = WHC in {950,951,952} AND space main is NOT a heat network.
+"""
+import json
+from pathlib import Path
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+from datatypes.epc.domain.field_mappings import is_heat_network_main
+from domain.sap10_calculator.rdsap.cert_to_inputs import (
+    _first_main_heating, _heat_network_standing_charge_gbp,
+    _WATER_HEAT_NETWORK_ONLY_CODES,
+)
+
+path = Path("backend/epc_api/json_samples/RdSAP-Schema-21.0.1/corpus.jsonl")
+certs = [json.loads(l) for l in path.read_text().splitlines() if l.strip()]
+
+hits = []
+for c in certs:
+    epc = EpcPropertyDataMapper.from_api_response(c)
+    whc = epc.sap_heating.water_heating_code if epc.sap_heating else None
+    if whc not in _WATER_HEAT_NETWORK_ONLY_CODES:
+        continue
+    m1 = _first_main_heating(epc)
+    net = is_heat_network_main(m1)
+    hits.append({
+        "rrn": c.get("rrn") or c.get("lmk_key"),
+        "uprn": c.get("uprn"),
+        "addr": (c.get("address1") or "")[:34],
+        "whc": whc,
+        "main_code": m1.sap_main_heating_code if m1 else None,
+        "main_is_network": net,
+        "net_standing": _heat_network_standing_charge_gbp(epc, m1),
+        "in_C6_half_cohort": not net,
+    })
+
+print(f"WHC 950/951/952 certs in 21.0.1 corpus: {len(hits)}\n")
+for h in hits:
+    flag = "  <== C6 HALF COHORT" if h["in_C6_half_cohort"] else ""
+    print(f"  rrn={h['rrn']} uprn={h['uprn']}")
+    print(f"    addr={h['addr']!r} whc={h['whc']} main={h['main_code']} "
+          f"network_main={h['main_is_network']} net_standing={h['net_standing']}{flag}")
+print(f"\nIn C6 half cohort: {sum(h['in_C6_half_cohort'] for h in hits)}")
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index 58c1150bc..fbdde3f29 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -68,6 +68,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _mev_default_data_iuf,  # pyright: ignore[reportPrivateUsage]
     _TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S,  # pyright: ignore[reportPrivateUsage]
     dimensions_from_cert,
+    fuel_cost_section_from_cert,
     _table_12_factor_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _table_12a_system_for_main,  # pyright: ignore[reportPrivateUsage]
     _is_electric_main,  # pyright: ignore[reportPrivateUsage]
@@ -9160,6 +9161,34 @@ def test_water_only_heat_network_half_charge_adds_to_the_dwellings_fuel_standing
     assert abs(total - 84.0) <= 1e-9
 
 
+def test_water_only_heat_network_half_charge_reaches_the_fuel_cost_worksheet() -> None:
+    # Arrange — the §C6 half charge must reach worksheet (251), not just the
+    # helper. A gas-boiler main (SAP 102, not a network) with WHC 950 keeps the
+    # dwelling on the STANDARD tariff, so `_fuel_cost` returns the real
+    # (240)..(255) result rather than the off-peak zero sentinel.
+    #
+    # Table 12 note (a): "The standing charge for gas fuels is added to space
+    # and water heating costs where the gas fuel is used for space heating" →
+    # Table 32 code 1 = £120. Plus the C6 half (£60) = £180.
+    from dataclasses import replace
+
+    base = _typical_semi_detached_epc()
+    epc = replace(
+        base,
+        sap_heating=make_sap_heating(
+            main_heating_details=[_gas_boiler_detail(sap_main_heating_code=102)],
+            water_heating_code=950,  # DHW-only heat network (boilers)
+        ),
+    )
+
+    # Act
+    fc = fuel_cost_section_from_cert(epc)
+
+    # Assert
+    assert fc is not None
+    assert abs(fc.additional_standing_charges_gbp - 180.0) <= 1e-9
+
+
 def test_mev_default_data_iuf_is_table_329_system_type_10_value() -> None:
     # Arrange / Act — SAP 10.2 Table 4g note 3 (PDF p.176) directs the
     # default SFP to "the appropriate in-use factor for default data from