You need a filter on each phone socket. It has to be the first thing coming out of the socket, otherwise it'll have zero effect.
If you're having problems, first thing to try is to plug your router into the master socket (it should have a fascia that comes off in two halves rather than one piece if it was installed by BT). Plugging the router into the master socket eliminates possible problems with the wiring to your other sockets.
If you're still having an issue, phone your ISP and get them to perform a line test and a diagnostics test. Advise them that you want to see the results of both tests. They should be able to email them to you.
Then post them on here and I'll tell you what's wrong.
It is worth noting though, that Tiscali had a 50:1 contention ratio when I last had experience of one of their connections. It wouldn't surprise me if this is still the case. Fighting 49 other users for bandwidth is bound to give poor performance, especially if the other 49 are in the process of downloading the internet.