Thank you all for your help.
I'm going to check out all of the tips you gave me; I'll admit that some of them sound like a foreign language to me but at least I've now got a starting point!
But first I'm going to do one more thing I probably should have done before I even posted my question:
I'm going to surf around and check out as many other forums as I can which have pay subscriptions.
I can't believe I didn't think of that until now. Doh!
Anyway, hopefully I'll find some which use PunBB and PayPal in the method that I want and perhaps I can get some hints that way. Failing that I can always contact the administrator of those sights and ask them how to do it.
I'll keep you all posted on my progress and if I can do it my way i.e. the easiest most non-techy way as possible, I'll post a mini-tutorial. Just don't expect me to be speedy about it; I've had this project simmering for a few months now!
Have a good evening and weekend everyone and thanks for your help.
Chris