Thanks to those....
....who took the time to think about this. There is only one extension in use for the base station for two cordless handsets in another room. There are other extension sockets throughout the house but there are no phones plugged in to them.
I'm going away on Wednesday for a week's hols in the sun, so I'll maybe give it some deep pondering in the sunshine and start again on return. Ho Hum.
The Ancient Mariner