Hi Eric, thanks for your prompt reply. My isp sent me an email saying they had requested interleaving be switch on. They did this because they missunderstood my complaint, which was about them and not the ADSL connection itself.
I can hear no noise on the line, and using the phone does not effect the connection. I have semi fixed the issue but it's a very strange solution, I have forced the netgear to use G.Lite instead of GDMT. It is now sycing at 3000 odd, but is stable, with speedtest being as expected. However if I put it back to auto mode, it picks GMDT, and while the connection syncs at 4000 odd, speedtests are 2/3kb sec and the connection can't last more than 1 minute.
It's very strange, until interleaving was put on, which I know was done last night, the connection was happy to sync at 5000 and give a good 450/500kb download.