Technically I woulod imagine that the vehicle owner retains possession, since he paid for the fuel to be in the vehicle in the first place.
If the garage are claiming it, i would suggest you deduct the fuel cost from their final account when you pay, and ask them if they consider that to be reasonable.
To be honest, as the fuel will be contaminated, it may not be of use to anyone, and you may have to accept your loss as part of the problem.
It's certainly a grey area, but I would not be keen to upset the people who's assistance you need - namely the garage - until the car is fixed and running again.