Presumably talktalk are been tight on engineer expenses.
I assume talktalk sell the service as "best efforts", that usually means lowest tier service, however it also should mean not artificially restricted.
I would send a polite reply stating that you simply want the service as is sold, basically for the line to run at the highest adapted sync rate, it is artificially capped excessively (12 db snrm with g.inp) and if they are not willing to follow due process of what is needed for a DLM reset then you want to leave your contract penalty free as they failing to fulfil those basic conditions.
Of course the question is are you prepared to change isp to get this resolved? and I havent personally read the t&c's for talktalk, just made assumptions based on what I expect to be in there.
There is naturally a conflict in the way the service is sold, it is sold as a rate adaptive service, but also sold with estimated speed's, and its a question of which one overcomes the other, as the latter allows an isp to only consider a fault if the estimated speed is not achieved.