We just charge ourselves £100 per hour wet, and have an account at the airfield. Whenever we fill up with fuel, we do it on the account. If one of us fills up away from base, we just knock the cost of the fuel off the £100 per hour (i.e. put it down as an expense against the flying costs account). If fuel price varies, then so be it, you win some and lose some, who cares about a few quid here and there it will work itself out eventually.
Anyway there is just two of us in the group so it is easy to manage. When I was in a group at Bournemouth we had to fill back to tabs and pay for the fuel ourselves. The problem with this is that one man's tabs is not quite another man's tabs and even if it is short by 5 litres (which is barely noticable) that is almost £10 these days, and you do get characters like that in aviation (tight arses).