Thanks for the advice so far. I'm not sure whether the ABTA "rules" come into play, as it was a flight booked direct, and the Thomsonfly website explicity states that flights from Bournemouth (and several other airports) are not included in the ATOL protection.
I have called the call centre again, and they are now saying it could take 10 to 28 days for the "errors" department to even acknowledge my email. Which sounds crazy - they don't even do an auto-reply to say "we've got your mail; it might take 14 to 28 days to respond to it" - which would be a lot better than the vacuum I have at the moment.
Interestingly, the call centre operator told me they are *not* allowed to put me through to a superviser to make a complaint about this system, as it all has to go through the "errors" department, who cannot be contacted direct - you have to wait for them to contact you.....!
Given the issues I had with a refund in March about a First Choice flight from Puerto Plata, I'm not sure I would fly with Tui / Thomson / First Choice again.... it certainly giving me a very poor impression.