I managed to get to site yesterday with Peter, and the asterisk is now back on line and working again.
The fault turned out to be with the BT router at Norchard (again!)
This time it was a new failure mode I haven’t seen before. It had the port forwarding rules configured, but for some reason had decided it didn’t want to actually use them any more.
I took the port forwarding off and put it back again and everything sprung back to life.