The cabling at the internal sockets is the standard 4way twisted pair I expected (hoped !) to see
Couldn't access the external box as it had an anti tamper seal that I didn't want to open, when I looked in the loft the cat5 cable was in view coming up through the cavity where I expected to see it
So, cabling seems to be OK
The fault on no3 socket was a bad connection in no2 socket...................... the blue to no3 socket had not made a connection through the sheath, easy enough to fix.
I know there are millions of these sockets in use with the self cutting wire clamps but I still prefer good old screw terminals
Thanks for the replies guys I'll see if I can end this thread by marking it as solved
A nice simple fault