Just going back to Kevin's point about a tax credits payment ... surely the source of the money will be shown on the bank statement??
Tax credits statements are send every year ... I presume it would be possible to get a copy from HMRC if you do not have it.
In my case I had to produce every bank statement and credit card statement for about 2 years .. updating frequently - for my personal accounts and for my limited company!!!
I had to account for lists of payments they identified ... but they just made themselves look daft because all were perfectly legit ... and blindingly obvious. It was annoying to have to pay the costs of going through all these hoops though
Even at the Final Hearing they were still asking me to expalin various items of sepnding in an attempt to prove I was frivolous - but only succeeded in looking very very silly.
Now his banks accounts and what they showed were quite another story

....
Kevin, stay calm, be honest, give short and to the point answers and if there are no documents, just say there aren't.
Hadenoughnow