If its fine via cable then its not Talktalk, try changing the wireless channel, encryption type and updating your wireless card drivers - might improve things.
FWIW I'm on BT and normally get a whopping 3.3 meg which has been stable for the last month or so, tonight it's at around 1.87 for the first time in ages.
Will do, cheers. A new router should solve this if it persists, right?