Treatment of Back Payments (PIP & UC)

Hello again.

Could anyone point me in the direction of legislation regarding how back payments are treated for UC?

My daughter received a back payment of PIP but UC say that, because it went into bank account A and was paid out of bank account A then she spent it so it no longer counts as a back payment, even though she has other savings (so she still has an amount eqivalent to the back pay - actually she has more).

If this is how it's supposed to be handled, that's fine but its not the first time UC have told her something that wasn't correct, so wanted to check.

They also stopped her payments for a couple of months (she updated her capital) and have now paid her three months worth of UC in one hit, following a review of her savings. Presumably this counts as a back payment?