EM2166 - Working the enquiry: partial closure notices: repayments
EM1609 explains that TMA70 Section 59B(4A) allows HMRC to withhold a repayment where there is an enquiry into the return. This continues to apply even if you have issued a partial closure notice (PCN).
The repayment does not have to be made until the enquiry is completed and a final closure notice (FCN) is issued under TMA 1970 S28A (1B). However you should not withhold repayment of more tax than is appropriate when considering the perceived tax at risk. If you withhold some or all of the repayment at the beginning of your enquiry, you should make a (further) provisional repayment if it becomes appropriate as your enquiries on a particular matter or matters are completed.