This is a subject, that I'm sure, we will see looked at by HMRC many times during our careers as the taxman wants to be sure we dont get something we shouldn't get !!!.
I know that many crew use their allowances downroute quite carefully and therefore have a surplus when they return home, so they can afford to pay the tax to fill up their cars i guess !!. But if you are asked to provide receipts then it is absolutely essential that correct spending of allowances is shown, or a guaranteed increase in tax on allowances will follow.
In summary for those who say they cant afford to spend all down route, for the period of the tax inspectors check at least, you cant afford not too!!