Yes I would certainly complain to Talk Talk as that is a very poor connection speed. They will ask you to check your connection speed through the master socket, with the face plate removed to ensure that its not a problem with the internal wiring. You might want to check to see what your modem as actually connecting at too as the download speed is always lower than the connection speed. Personally I think that there has been a significant reduction in the broadband service in the area recently. I am hoping to get a reply from BT with regards to this during the week and will let you know what I find out.
All the best