So after being persuaded to let them send another engineer out instead of raising an official complaint, we have now (yesterday) had another engineer's visit. He has advised quite clearly that "BT" need to be forced to sort the issue, as he believes it is definitely the line and is not broadband related. This seems to align with what you have advised.
So before I had a chance to call plusnet today, as I pay them the line rental, I received a text message.
Plusnet: I firstly apologise for the issues with your service. As no fault could be found on the engineer visit, please try an alternative router and monitor the connection again or call us on 0800 432 0200 to discuss getting a router from ourselves.
So I called them.
After five engineers visits (two with a truck ready to renew the drop wire) and two who were prepared to write down what type of fault they had found on the line, the intermittency has again lead to it being signed off as "no fault found"
After confirming that they cannot help with third party routers, I was advised that I would have to buy one from them as it was out of warranty. I refused and said I would simply raise a formal complaint instead. He was quite keen that I didn't do that and told me he would ring back after he had spoken to a manager, and hopefully get the router replace FOC, bearing in mind the fault has been going on since mid April.
The manager did in fact very kindly offer to replace the router FOC......if we were prepared to sign up for another 18 months.
You couldn't make this stuff up, and I certainly won't be put off any longer from making a formal complaint.