Just to add some balance as so often only disappointed users bother to review:
I've been with Plusnet 9 months. Speed was slow (like BT previously) at first, but after talking to them they 'tweaked' stuff and it got much better. Phone them with a question, and you get a fast response by people who know what they're talking about (OK - raising an online 'ticket' is slower).
They currently do 3 months free if you sign up by March 28th here: http://www.plus.net/refer
using china1 / refer3 so that can't be bad on top of their already decent prices!