Just an update on this. We've had a visit from an Open Reach engineer who ran some tests and, according to his equipment, from the wall socket our product is fine. The visit was coincidentally at a time when upload speeds were on the floor (it was a Tuesday visit and things had deteriorated badly over the previous weekend). He requested a new Smart Hub which arrived Wednesday and we installed it on Wednesday night.
Since then, I think the situation could best be described as acceptably bad
. So, upload speeds reliably in the 3-10 Mbps range when tested, with just two tests where the final score was below 1 (we've done many) and, more importantly it's not stopped me working, doing video calls and saving my work to the corporate network. I'll keep an eye on it over the next couple of weeks.
Incidentally, we have excellent 5G coverage, locally, and that has been a real bacon saver (seriously, the only thing that would beat EE's 5G, in good coverage, would be FTTP and higher than BT's base product offering).