TAG was untaxed - obviously supposed to be actuals with notional limits of £15 for breakfast, £25 for Lunch, £35 for Dinner and £10 overnight outside the UK.
But dependent on which aircraft/account you were on etc
Crews were randomly selected (although advised beforehand) that they would be audited and would have to provide receipts to go with their claims. All other crews are supposed to retain receipts for 6+ years incase HMRC asked for them.