Checked out further today: online account details and correspondence indicate the service was switched to fibre in July 2018, with expected speeds of 40Mbps+. Called BT to enquire about the slow speeds. They ran some tests, confirmed the speed was due to a fault, and made an appointment for an engineer investigation next week (promising it will not be charged). Roll on Monday afternoon!

So it appears you have a bridge tap fault (� is if it was anything else, they would have requested an appointment)
... be interesting to see what speeds you get after the wiring issue has been sorted.

So the BT engineer came, fitted a modern master socket (don't know what else she may have done), and now download speed is 50Mbps. Result!
KP