CTM97500 | CTSA: group payment arrangements: closing date
From HM Revenue & Customs · Company Taxation Manual
The closing date, in relation to each group payment period, is the later of:
the date when all the companies covered by the arrangement have:
filed their returns,
or
had their tax payable determined under or 37 (see onwards),
and
the last statutory filing date for the return,
for all relevant accounting periods.