I think I can guess the answer to this, but wonder whether anyone has actual knowledge.
Client was getting TCs as single parent. Partner moved in. Client told TCO. Single claim terminated with award showing an overpayment. (Which is disputed, but that's another matter!) Award says "as we are no longer paying you Tax Credits we cannot collect the amount due by reducing payments. We will contact you shortly to tell you how to pay back the overpayment."
In fact the client has already submitted a couple claim and they will be entitled to TCs on this basis.
Will TCO be able to recover the single claim overpayment by reducing payments on the couple claim? COP26 says nothing on this, but is it covered in regs. etc. anywhere?