Thanks for the advice Genghis. Funnily enough I asked Sibson that question today whilst booking another slot for Saturday. They seemed very flexible and said that usually they'd expect a minimum of 2.5 - 3 hours flying, however if on a weekday and they weren't busy then they wouldn't worry about that too much.
Buying a share is certainly a consideration, but as you say I won't look at that until after I've got my PPL. One step at a time! I've got my medical certificate after much prodding and poking, so PPL next then I'll consider my options from there. Would imagine I'll want to add night flying at some point too.
Roll on Saturday (and good weather!)