A colleague had same thing with talk talk, check first thing in the morning or before 5pm and it’ll be lightning fast. Apparently what they do is share the 16Mb line from your nearest exchange between the number of users the majority come on after 5pm when they get home from work.
He complained repeatedly to talk talk and they kept saying we’ll fix it 3 months later (I wouldn’t have lasted that long) he told them to sling their hook, he went with PlusNet and he says he gets perm 8Mb now. They even sent an engineer and said he must be in the house in the morning so he tool they day off work and stayed in, Eng tested the line and he got 8Mb continually and said there was nowt wrong with it, that night (and almost every other) he said he was lucky to get 1mb, sometimes down to 0.3mb.