Simple way around it all really.
Describe in a letter how you advertise and conduct trial flying lessons. Mention any disclaimers you have, when you carry passengers, membership etc. Send three copies recorded delivery, one to the Minister of Transport, one to the CAA and one to John Prescott (well it will give him something to do!)
If anything ever arises from all of this, which none of you will ever solve here because there has never been a test case, you can turn around and prove that you notified the authority of your actions and they did not comment. Balls in their court then isnt it?