To cut a long story short, after multiple phone calls and three visits by engineers, the problem was solved, although in the interim we completely lost service for a while.
The third engineer said a 3rd party engineer had taken our fast connection for someone else and put us on a slow one.
Thanks for the update @msjhaffey, I'm really glad to hear that this was resolved in the end.
I appreciate that the total loss of service is far from ideal though, and you should receive automatic compensation on your next bill for the time that you were left unable to connect.
Thanks again for your patience here too.
Peter
And here's the ironic thing: on the four occasions I've tried clicking the link to claim for compensation, I get this error:
I always have a little chuckle about web pages that are not working in order to "improve service".
Getting the compensation proved fiddly. It took three phone calls and then, instead of applying the compensation to my account, BT insisted on sending me a card (a kind of card with cash on it) which I had to register and then get a PIN to use.
Just a hassle.