COM123012 | Repayments/reallocations: repayment cancellation: stop repayment (Action Guide)
From HM Revenue & Customs · COTAX Manual
To stop a repayment before the cut off time (4pm) for processing of repayments, consider steps 1 - 3 below.
The cut off is normally the same day as the day the repayment is created but it is 4pm the next working day if the repayment is created on a Saturday, Sunday or a Bank Holiday.