Suilvenms, I can't fault find from a distance and I am not trying to protect your supplier at all,but it is possible from what you say that the fault is with you. There is a limit to how many phones you can connect to a line before they stop ringing, depends on the phone type but three would be a typical maximum. Connecting even one phone on a broadband line will drop the speed, every additional phone will drop it further. With no phones connected I get 8MBPS with one phone 6MBPS and with three phones 3MBPS and I am close to the exhange.
Faulty filters on your phones can have even more impact and they do go faulty and some are better than others. A faulty filter on any of your phones can drop the broadband to nothing. Over the past few years I have had to replace two or three of them.
I am an engineer so can tackle them at a technical level, but even then I admit it is often difficult to get an answer from them and many of them just don't care.