Not sure who is confused here, as I am only dealing in known facts (and by the way I am not trying to have a go at you, my initial reply to you was rather a generic observation to all taking part in the discussion). I do complaint myself a bit and think too that raising a valid point in the forums is a good way to share a problem and find a solution.
I don't know the nitty-gritty of your case, but what I do know is that you said that "I have already done everything (bar the official complaint to OFCOM) that you have mentioned as the way forward." Would it be reasonable for me or indeed anyone to assume that you got a deadlock letter from Plusnet, if you followed everything on this page at http://www.plus.net/support/service/policies/complai...
? I think so.
The deadlock letter would have given you in writing the final position of Plusnet on the issue, and it would have been up to you to make a decision on whether to carry on staying knowing that they cannot do anything to resolve it or to move somewhere else.
So if you don't have a deadlock letter from Plusnet, it might be a good time to ask for one. This could be a final push they need to resolve your problem.
In any way, you will get in writing their position, and you can take them to court or use it to complaint to CISAS, or even post it here for us to see what they said, so we can discuss if they have really done everything possible to help you.
So, I am not advocating in giving up just yet, but I do advocate to get a deadlock letter and then make your decision based on what it will say.
As for intimating that you are lying, well, I find it surprising that you have been unhappy with the service provided by Plusnet for many months yet still choose to stay.
So if you genuinely want to stay and help your ISP to sort this problematic issue on your line, perhaps it's a time to take a step back, let them deal with it and at the same time proceed with any official channels still available to you to get the definite answer on whether it can be resolved by Plusnet or not.
I wish you all the best with this.