Thanks for taking the time,
Luc Lion, I think I have it now
In the various cases my client flew me and one other person and I personally never saw any paperwork. But I think you're right, it probably wasn't legal, as I understand it now because a) no AOC b) probably no CPL c) I am virtually certain money changed hands against a receipt.
I am given to understand that my client proceeded on the basis of seeing a pilot's licence and an (unspecified) insurance certificate.
What's really getting through to me here is that unless a client is really really clued up to the point of having either specialist advice or knowing a lot about general aviation themselves, it's really really easy for a pilot to give the impression of providing a legal and (reasonably) safe service when one may not be getting either.