CISAS

[#1]

If you feel that you are not receing the correct level of service that you think you should expect from Plusnet then make a formal complaint and if you are not happy with the complaint then insist that your case is refered to the CISAS.
plusnet will try to fob you off but as soon as you state that you want to go to the ombudsman in a lot of cases they will deal with your complaint
Plusnet have to pay each time that  a complaint is passed to the CISAS - but it is free to the customer.

[#2]

You don't need to go via your ISP to log your complaint  with the ombudsman  http://www.cedr.com/cisas/make-a-complaint/

[#3]

You need to raise a formanl complaint with your isp first and if they dont deal with it within 8 weeks or dont dwal with your claim to your satisfaction then cisas will become involved
http://www.cedr.com/cisas/frequently-asked-questions/
section 5 is the applicable point in this aspect

[#4]

Yes i already knew about that, I have had to use the services of cisas twice in the past