[email protected] blast anyone; if you phone the BT 0800 help line, they'll guide you through all the checks. Be very patient, and be prepared to repeat thing thye haven't understood and vice versa. My system is also connected to the master socket and they have sorted out slow speed issues a couple of times recently.
if you want to check some things first:
log onto the router's home page and check what it says about speeds.
If you use a filter in line with the connection of the router to the master socket, it's worth uncrewing the front plate of the master socket and plugging the filter into the socket that's behind the master socket - then see is speed increases. Also try a different filter; they can fail.
However the BT people will guide you though all this.