Thanks Lee,
Lee once again hit the nail on the head - and identified the problem as a network issue.
It was caused by having a PAW unit plugged in to the wired network (running temporarily in ground station mode) alongside the OGN-R unit.
It had been working fine for months but we recently had a rare power trip so both OGN-R and PAW restarted together.
There was an unfortunate interaction between the PAW and OGN-R sharing the same network and it resulted in the OGN-R unit not being able to see/resolve any external names/addresses even though all local network addresses available. So could not connect to NTP or the Grid.
PAW now not connected to the home network, and OGN-R is connected to Internet, NTP and Grid. All good. Hopefully will be able to see return of Grafana heartbeat.
Thanks again
Best wishes
Chris