Was going to post an update last night, but we had a power cut!
Anyway, this was the reply in the newsgroups:
I have configured it for the 10 meg now please run the test again and
let us know how you get on. The modem you have should be fine with the 10
meg.
Strange how some people get told the ntl 120 will need replacing, other get told it will be fine.
Did some speed tests yesterday evening, and just done one this morning, and getting 5Mb:
It's almost constantly that speed on any speed test, and if I'm wireless or directly connected to the modem.
Did manage to post this back on the newsgroups last night before the power went.
Wait to see what they say.