It's all sorted now!
The two upstairs extensions have been connected into the master socket internally via the punchdown block and a microfilter fitted to the phone on one of the upstairs phone sockets and the modem has been plugged straight into the other extension.
The master socket has a microfilter plugged into it and his downstairs phone and Sky Box plugged into the micofilter via a 2 way adapter.
Everything is working, phones and Broadband
I also found some info on microfilters:
http://usertools.plus.net/tutorials/id/13