I went through exactly the same with TTnet, in fact in the end, you feel like a hamster in a wheel. I'm on a complex and neighbours were getting double or treble my speed, but they still insisted that it was a distance from the exchange problem. I ended up taking my IPad into the engineers department showing speed-test results over a 24 hour period. After this they moved their attention to the cable that runs from the phone terminal box which is situated on the complex perimeter and blaming the cable that runs between this box and my house. It was my understanding that its their responsibility to bring the service to my door and my responsibility for any internal connections. They weren't keen on this claiming that the signal was fine at the terminal box in the garden, thereafter it's down to me.
I got the electrician to but 70 metres of the best quality cable along with the same amount of armaflex sheathing. We then dug a trench and connected the cable at the box at one end and at my modem at the other. My heart sank when I did another speed-test and it still showed less than 1 meg. Once again I called customer services to explain what I had done, no problem was the reply from a helpful agent at their end, now you have replaced the cable I can turn up your signal. My modem reset and I'm getting around 5meg. It's still not perfect as they are still digging up all of the roads around me to either install new drainage or electricity cables but it is a hell of a lot better, in fact yesterday we watched the Liverpool match totally uninterrupted on Sky while keeping our eye on the Man U game which was running on the Ipad.
All I can say is persevere, eventually I'm sure you will wear them down. Good Luck.