The length of lead shouldn't be a problem, the range for maximum speed is about 1.5km, and ultimate range is approx 8km wire length from exchange, chance that 11.5m pushes you over 8km is remote.
Check you have an adsl filter between the extension lead and your telephone socket. If you have more than one phone socket, each must have an adsl filter installed, this includes sockets with modems and fax machines connected.
Are you connecting with a separate adsl modem or an adsl router?
Bookmarks