In reply to:
Well! I'm mesmerised by all this technical talk!!
Apologies - point taken! Yes, you can definitely say that you've transferred to TalkTalk's 'up to 8Mb' service, and that the improved technology they use on this service should be able to achieve a better speed than the 2Mbps you had before. Unfortunately they currently appear to have configured the new service so that this is not the case. The 'noise margin' is the parameter in question. This is set by the ISP as a compromise between line reliability and speed. The higher the margin then the better reliability but the lower the speed. What I think from your stats is that they've set this far too high.
You can either ask them to lower it, or a possible alternative would be to ask them to set you on a fixed 2Mbps on the new service. The former gives you the possibility of exploiting the improved technology to go to even higher speeds. The latter is probably simpler to ask for.
Hope that makes more sense!