Are you still permanent now molgrips, not contracting? – It’s your direct employer you’re invoicing I take it
The only, rather far fetched, idea I can come up with is that they want proof that you took the flight/train/class that you’re invoicing for- i.e. that you didn’t book a full price refundable or business/first fare, then cancel that and book a cheap replacement, but still invoice them for the full amount using bank statements or similar.
Been told about this happening, but I’ve no idea whether it would ever work or not- hearsay only.