Hunch. On 14 October Ofwat named 15 December itself, and new bills from 1 April 2027 depend on that decision landing.
The 15 December market resolves on the same Ofwat page, confirmed by The Guardian and BBC News: YES if the final determination for 2025-26 is published by the end of 15 December 2026, and NO if it is not.
We examined six selected dedicated annual in-period ODI draft/final rounds from 2016 to 2023. All six reached final determination within 49 days and by 15 December. Substantive changes exceeding £1m are documented in 2022 and 2023; the available evidence does not establish a complete change rate.