Thanks, Garry, that's very helpful and informative.
Modem firmware: unfortunately, it was confirmed by IDNet that I (and, as you say, they) have no way of accessing the modem at all, hence I can't know anything about firmware. Seems an unnecessary lockdown by Openreach, and similarly unnecessary to withdraw support from a device which is still under warranty (to Openreach at least) from its original manufacturer, but I guess that's their decision. I may end up having to get a Draytek Vigor as you suggest, but I'd prefer to avoid it if possible.
I don't know what type of cabinet I'm on, and my attempts to find out today fundamentally failed. The PCP cabinet at the end of my road is easy enough to spot (and in pretty poor shape, I might add; leaning back and with the lid not properly on!), but no sign of an FTTC cabinet anywhere. Wandering around, the nearest cabinets I could find of about the right size were a good 70m away, and none of them had vents which I believe the FTTC ones always do (according to the pictures at Kitz), so I'm a bit in the dark in that regard. Would be nice if Openreach maintained a publicly-accessible database with that kind of info.
Where is the BT wholesale tester that would show my profile? The only one I can find is:
http://speedtest.btwholesale.com/ and that just tests my speed (and seems to overestimate my download and underestimate my upload speeds, compared to other testers). If I go for further diagnostics with that tester, I get the test error message, and no profile info.
I'm very interested in the single threaded issue; could you or someone else please explain it to me? I understand threads in the context of processes, but I don't know what the connection is with download speeds? From what I've seen elsewhere in this thread, a single thread download speed significantly below the multi-thread download speed is a problem? Because that's what I *always* see; even when my multi-threaded speed is okay (i.e. > 50Mbps), my single threaded speed is never more than about a quarter of that. That's about the most consistent/reliable feature of my speed testing in fact. I'm wondering what the significance of it is, though?
IDNet tested the line (not sure specifically how) and said it was okay, and that it was synced at 64Mbps. I don't think they actually tested the speed of the line and saw a real 64Mbps download speed, or at least that's not what they told me.