Next time you prod them, ask about the compensation scheme. I don't know if its still active, but it lets them know you mean business. Complain by letter. Do it on the PC. This lets you append copies should anything "go astray". Make a log. To date, it will be from memory, but any further contact should be logged.
I don't know if having one supplier for everything is as good an idea as it once was - as a recent example, my No 2 son had a problem with his phone (Sky) and duly reported it. The usual tests were done, indicating a line fault. The Openreach man was there later the same day. Due to the weather and the site, he needed a buddy with a hoist, but it was fixed by 8.30 the morning after. It could be that once a problem is pinpointed to BT plant, the large corporate customers such as Talktalk and Sky have the ability to chase it more effectively than an individual can.