Take the £11.75 one off payment for a monthly contract option. I have several friends on PlusNet and there have been a few problems in recent weeks with their upgrading processes, mainly with their new servers but everything seems to be working fine now.
Plusnet have extensive self help online and you can find answers to most problems there. (Or just post here on the forums!) In all honesty I can`t say that I`ve used their telephone helpline all that often and probably like others it`s been a "lazy option".