Ok, to continue (sorry for the interruption, but there are three types of people the Experienced Traveller does not argue with: Customs/Immigration Officers, Lounge Dragons and Hotel Maids) ...
So, I'm in Punta del Este, with limited internet access; when I managed to check my email (yesterday/Tuesday evening), there was a message, sent on Monday morning, from Opodo (in French, from info "at" opodo.fr, for some strange reason) saying that my flight, MVD-EZE on Thursday was 'cancelled' and that I have been rebooked on the 18:30 MVD-AEP flight (arrives 18:20).
This, of course, is completely useless (15:40 departure from EZE)! I replied to opodo.fr, explaining my situation, but received no reply. So, I emailed BQB directly (I had their email address from the previous notifications of changes to the outbound). Incidentally, I also asked, in my original reply to BQB, if my return was still OK; they confirmed that it was.
I've now received a reply from BQB saying that the information from opodo.fr is incorrect, and that the MVD-EZE 10:30 flight is as scheduled. They will let me know if anything changes.
So, in summary, BQB is generally a nice, elegant airline (and they are expanding their route network - already serving Rio, Sao Paulo, Florianapolis and Iguazu, and they will soon be buying some Airbuses to serve Santiago de Chile). There was never any reason given for the cancellation of my original AEP-PDP flight; however, I suspect it was simply due to a very low load factor (the later flight which, presumably, had all the other pax from my original, had only 25 pax on board - the ATR-75 holds up to ~70 pax).
BQB have an office in town, here in Punta DE, so I'll probably pop in to double-check either later on today or just before I board my bus to Montevideo tomorrow (the office is at the bus terminal). As an emergency option, I guess I could always get an evening flight to BUE on 30-OCT, and take an overnight there.
I can't fault the friendliness and helpfulness of the BQB staff (both in face-to-face contact and via email). Another poster described it as an 'Old Style' airline: well, despite some peculiar (and mildly frustrating) schedule changes, they certainly maintain 'Old Style' manners and courtesy.
Here's hoping everything works OK, and that I get back home without too much further angst!
-- Henry
PS: I'll be chasing up Opodo when I'm back home, just to know why they sent me this incorrect notification.