This is a tricky one, Colin. I myself, am a bus driver with a P.C.V. licence (obviously) and even I, or should I say, we have trouble with hiring minibuses. The main problem is that some people just do not have the correct licence to drive these vehicles according to when they passed their test. If you only have a licence to drive vehicles with 8 seats, then you are stuffed basically. If this wasn't enough there are very few hire firms which actually hire out minibuses due to this hitch and also insurance is very expensive on this. I personally do not know any firms who hire out minibuses on a self-drive hire basis.
Hiring out a minibus or coach with driver however is child's play. What to do is ring around all your local coach firms and tell them your requirements, and let them do all the work. You will not have to drive, and the fuel costs are all theirs. Don't know what prices are like in your neck of the woods are, but here a return trip from Shrewsbury to Manchester Airport is about �225. This, of course, is all in, so you would not have to worry about hiring for the whole week. This is what I would do myself, but if any other ABers know otherwise you know what to do.
Jonny