I run a normal BT extension cable(10m), then plug in the filter and use the standard 1m rj11 into the adsl modem/router.
Same here, except I use a 15m one - I really don't think it makes that much difference if you use a high quality cable such as Belkin or something but the cheaper ones are probably OK aswell. One thing to note is that BT say (and after testing I agree) that UK ADSL broadband always works better when you connect your router to the master socket - the one where the physical cable from BT comes in from outside into and all other sockets in the house come off. You can usually tell which one this is as it has a removable top half with a phone plug in it that is used by engineers.
Hope this helps,
Seb