Thanks for your comments, there seems plenty of encouragement for me to pursue a fault if I can get VM to look at it.

The master socket comes underground to a blanked off box in the hall, with no traditional test socket behind it. From there a normal looking telephone wire is led about 1 metre to the study, where it terminates in a box with an NTE5 plate on it, the wires connect to the rear of this plate. I have a filter plugged in there to which the phone and ethernet cable plug into. The extensions were wired to this "second box", and this is where I have disconnected them. I'm not sure what a star junction looks like.
I don't have a phone-connected alarm and my sky box is not connected to the phone line at all, ever.

Many thanks for your help.

RobertoS,

Where do you think I should start with VM to get them to agree to look into a possible fault?