I use TalkTalk and had similar slow speed problems of barely being able to get 1mb on a supposedly 10mb package. One thing that I learned from that experience was the constant false information I was getting from TalkTalk about the line being okay. It wasn't and eventually I was able to get 6mb after quite a bit of repair work by BT OpenReach and TalkTalk's own technical team.
You may also find that the engineer TalkTalk sends is someone local on contract to TT. The engineer who called to check my system changed the router, and still could not get any better performance, did further checks then simply left it to TalkTalk to sort out the problems.Regarding the engineer's fee, I was originally quoted £99.99 then £29.99, and when the engineer arrived he insisted it was £49.99 before he did any checks, and wanted me to agree and sign to this. I refused and got it down to £29.99 which I had agreed with TalkTalk. Perhaps lucky, but due to the engineer being unable to rectify any faults, TalkTalk cancelled any charges.