VENSAV4250 | Grant-funded installations: connection or reconnection to the mains gas supply
From HM Revenue & Customs · VAT Energy-Saving Materials and Grant-Funded Heating Supplies
The reduced rate covers situations where a qualifying person:
is connected to the mains; or
has been disconnected from the mains,
and the connection or reconnection is paid for under a grant-funded scheme.
This is covered by the VAT Act 1994, Schedule 7A, Group 3, Items 3 and 4.