Water bill projections for 2027-29 cannot be reproduced from the repo #2384

The 2027–2029 values of gov.economic_assumptions.yoy_growth.ofwat.water_bills (4.6%, 4.5%, 4.5%) cannot be reproduced from the repository.

How they were made

policyengine_uk/utils/water/forecast_water_bills.py (#1224, #1239) takes per-company bills before inflation from the CCW breakdown of bill increases 2025–30, weights them by customer numbers, and adds a hardcoded CPIH path (134.0, 139.0, 142.2, 145.2, 148.2, 151.3 for 2024–2029, vintage unrecorded).

Problems

• The weights and bills were read from ofwat_increases.csv, which .gitignore (**/*.csv) excludes. It was never committed.
• The CCW page says its table shows the increases proposed in Ofwat's PR24 draft determinations, not the final determinations published on 19 December 2024.
• The CPIH path predates the current OBR forecast in yoy_growth.obr.cpih.
• The script writes test.csv to the working directory and prints the result instead of writing the parameter.

2025-26 and 2026-27 now use published figures (27% from Ofwat's 29 January 2026 statement, 5.4% from Water UK's 28 January 2026 charges), in #1863.

To do

• Rebuild the 2027–2029 projection from the PR24 final determinations, with company customer numbers from a cited source, committed as a tracked input (with a .gitignore exception).
• Take CPIH from yoy_growth.obr.cpih instead of a hardcoded path.
• Write the parameter values, or a check against them, from the script.
• policyengine_uk/utils/create_ahc_deflator.py reads this index; rerun it after any change.