OK, some progress!!
As I said, there are four LAN cables running into the house.
Three of them are down for reasons unknown, but one is working - so I have repositioned the original router and now have a stable connection.
I have no idea whatever as to why the other three connections went down, but at least I know what the issue is now.
Huge thanks for the replies - you confirmed that I am not mad, and made me check my work - which was important as I had it wrong!
Next thing is to fix the broken connections, but at least I know what the problem is now.
All the best
Bruce
