Thanks everyone.

I didn't think it was right. As Ziz says, I don't see either what BA reckons could constitute an involuntary downgrade if you follow its logic!

I didn't get to speak to anyone apart from at the bag drop at ATH as it was the usual BA irrops zoo, so I've only had "assistance" from the BA Twitter gang at the moment who are, doubtless, trotting-out the party line to unsuspecting dupes.

The flight on which I did travel only had three rows of CE (A320) so I don't imagine I was the only one affected as the original flight was a 767. I assume this was to get the most ET passengers in - or it could have been the new ex-London catering that meant this was all they could cope with...