Hello all,
After getting frustrated with cables, I called someone (on the phone that works) who worked for BT.
He opened the socket, connected two wires at the master, went upstairs to the phone socket and had to move two wires in that (different colours), and it's now working.
Odd thing is that the phone socket in the bedroom has never been touched, so unsure why that had to be changed too, but it's working, and my skybox upstairs can now call home
Thanks to all who suggested things, perhaps I'm just not cut out for telephone cabling...!