I have been with Plusnet for over 3 years now and no problems, possibly because with my contract didn't require all bells and whistles, I just pay for line rental and the internet and at the moment I'm on a 12 month contract paying just under £24 per month. When I renewed in February the renewal price was going to be £29 p/mth for 18 months, I told them I just wanted 12 months contracts, no problem. Then I told them seeing as how they were advertising contracts of only £18 p/mth to new customers why can't I pay the same, the reply was something along the lines of that would be impossible so after much haggling we settled on the above price.
For what I'm using I certainly wouldn't consider paying any more than £25 p/mth, and as most other companies advertise their standard contracts at about the same price why should I?