Having gone through the same routines as yourself, I eventually got what I was seeking from TalkTalk by going direct to the CEO Dido Harding.
Problems that were first denied, where then found with the line and at exchange level, and this involved TalkTalk and BT over a prolonged period, I should perhaps advice, that TalkTalk might use a local sub-contractor and not their own employed engineer, so someone might need to pay for this service. I contested the charges TalkTalk wanted or suggested at the first opportunity, and won my case.
You could do some checks yourself, by going through the router using 'admin' as the password and username (assuming your are using a TalkTalk supplied router?). Go through the selection in 'advanced' and that should tell you how your service is working 24/7 (Try to keep print-outs on the results for comparisons). There are a number of other tests that you can do yourself with the results being reported back to TalkTalk engineer's, and this includes going through TalkTalk's own speed checker. TalkTalk also use http://www.speedtest.net for comparisons.