Hi,
Try
thinkbroadband's ISP Search
here, which will also show which ISPs offer fixed rate ADSL.
Which exchange are you on? If it's been unbundled by Easynet, then I would highly recommend UKonline's LLU packages for a great fixed rate service.
Another option, what router do you have? Kitz reports that the Netgear DG834 and DG834G can apparently hold on to lines right down to 0dB SNR (Westell 6100 and D-Link DSL-G604T are also reported to be good for low SNR lines)
see here.
UKonline LLU

please use you're, your and they're, there, their and to, too and its, it's correctly:
Whilst you're crawling along in your Lada, they're flying there in their Learjet,
proceeding to down too many glasses of champagne from
its galley and commenting "Look down there, it's a skip on wheels". 