The saga continues. I plugged my work laptop into the master socket and even tried the "test" socket for a while at the weekend. Trouble is when you do that, you cut off all the rest of the phones in the house!
Anyway, on Sunday I got a call from BT faults department to say that they had changed some settings and that I should get faster speeds soon. Sure enough, after about 10 minutes, the download speed shot up to around 3.0mbps!!
I monitored it for a couple of hours and it hovered around 2.5 - 3.0mbps. By the evening, though, it had droppped back below 1.0mbps and has been around 0.5 mbps today.
I have been back on to BT faults and the latest is that I will hear from an engineer in the next 24-48 hours. I'm not holding my breath....