Thanks, Steve ( and to all who replied). I will follow up with AOPA
straight after the holidays (already have student membership).

Unfortunately I really messed up by not doing the pre-vetting of the AME's I
should have done - went to the one highly recommended by the flying school.
I think in retrospect he was recommended because he did such a quick and
perfunctory once-over that he would never have failed anyone who was still
breathing if they hadn't stubbornly insisted on being absolutely honest ...
at that point he just said "I have to defer this - the FAA will contact you
to ask for further details" and sent the form off without any supporting
documentation at all.

I did explain beforehand that I wanted to discuss my condition before taking
the medical, but his staff still got me to fill in the application form
before seeing him and I then discovered that the act of filling the form in
committed me to making my application there and then ...

Like I said, basically my fault for being so dumb. But it sounds like, from
what you say, I should get started on supporting documentation and send it
in myself without waiting for a formal request from them ... or does that
just confuse the process further?