Hi Toby and welcome.
This isn't formal legal advice (which you should seek) but I would think that they would have a very tough time proving that you owed them anything if, as you say, nothing was mentioned before or during the trip.
In my view, it would be reasonable for you to assume
that as you were asked to 'help out' and at short notice that there would be minimal to no cost to you.
It could of course just be a misunderstanding - the hotel(?) may be billing you now but why weren't you billed at the time? Did the team simply forget to pay your way? It might be worth a few questions in their direction but be firm and don't give in too easily.