http://www.clarity.it make a modified BT faceplate that gives you separated
filters phones _and_ ADSL out the back. I use this very successfully. What
you do is the following...
1. Replace all your 3-core (or at least the leg to where the computer will
be) with good 6 core (or CAT-5/6 if you have some or can get some cheaply).
2. Run the regular 3 phone wires as are they are now.
3. Use two of the remaining wires to run the ADSL.
4. Where you want ADSL, replace the faceplate with a "double phone + ADSL"
faceplate (Clarity and others sell these) and wire accordingly.
5. Plug your computer into the new ADLS socket and surf away!
WARNING: Obviously leave the master faceplate unplugged whilst you're
wiring up the other ends of the extensions.
Alternatively, if you must stick with the existing 3-core, you will have to
put filters everywhere because, as others have said, the output from a
standard ADSL BT master socket faceplat is "phones only".
Paul DS.