Thanks jedi98, that seems useful. I will watch the web page for a while to see if it resets.

Whilst watching the router web page I note the System uptime and Wan status times fully reset to zero with the fault, so presume the problem lies in the router or its power supply?

Yes, I would agree. It's rebooting so it's either a router HW fault, corrupt firmware or PSU fault. If you are going to suspect hardware then PSU is the 1st suspect.