@Dobby_
These are my settings which I guess is why dashboard is ok.
I decided to try setting up a monitoring tab for thermal sensors and selecting Advanced/Reset data, I assumed this would just reset the thermal sensor data, but it reset all RRD history. Documentation does say it clears all files! At that point, the thermal sensor tab started working and ran fine overnight.
I now see this is stored in /var/db/rrd so maybe if had deleted /var/db/rrd/system-sensors.rrd it would have fixed the problem and kept my other history, too late now!