Phoned support - call picked up immediately, spoke to someone called Chris, really helpful, said he would run tests and call back in 5 minutes, which he did, and then logged the first phase fault with BT, he said it might not work but give it 24 hours and then call back.
This didn't work, so called today, spoke to another chap (can't remember his name), right to the point and he raised it as a stuck BRAS.
Speed has returned now.
Really impressed - both people who I spoke to were right to the point, no messing about at all.
Zen 8000 Pro