Pirate,
OK, sorry, I didn't know that. But I don't think it alters the medical issue.
Either a trial lesson is what it says - and you don't need an AOC, and the instructor needs yearly medicals (according to the CAA's own rules).
Or it's a pleasure flight, you do need an AOC, and the medical requirements are of course in accordance with commercial flying requirements.
Since at the moment a trial lesson is considered as flying training, what happens in the future or in a hypothetical court case is not relevant. I'm trying to establish what bona fide flight instructors, carrying out real flight training, need in the way of medicals. And I think that's now clear. I've printed out the document which sierracharlie kindly directed me to, and sent a copy to my AME. So many thanks to everyone, and I sincerely hope this matter is now sorted.
Phew! Why is everything to do with flying so complicated?