So, in true MSE style, I'm trying to minimize my spend here
I've just had Sky BB installed to replace an abominably oversubscribed VM connection, and this entailed a brand-new master socket.
The BT engineer wasn't particularly helpful and decided to stick the socket right next to the front wall, which isn't where the phone lives. I've got the end of a reel of CAT-5e cable, so I used this to join to the cable going to the VM master socket.
This appears to work just fine, but do I need to buy a slave socket to replace the VM master?
It's wired through a Philex ADSL filterplate, so I'm not worried about introducing interference into the line speed.