Thanks to all who replied to my question.
You'll be pleased to know that I got it fixed

The discovery of the actual problem was purely chance. I
was opening the extension socket box for about the fourth time
to check my connections when the pin 3 wire(orange with
white stripes) popped out - but not at the connection. Looks
like the wire had severed at the point where the outer sheath
of the telephone stopped but the point of breakage was just
enough inside the sheath to hold the wire in there so you
couldn't see that it had snapped.

Stripped the wires back and reconnected...sorted!

Thanks again.
Ronnie