Nest thermostat heats randomly

[Andycbates]

My nest thermostat has worked well for at least two years, but suddenly it will randomly turn the heating on, although the nest is set with heating off (as it is summer). Im in the UK and have checked the valves and heating wiring and they are all sound. By "randomly" I mean  it seems to heat at 9am but this time seems to be inconsistent.

[Andycbates]

thanks , have done firmware upgrade , hadnt thought of reset but have power cycled , cannot find any schedule conflicts , not totally sure of the concept of an override so ill look for that next and factory reset seems like a good option ,thanks ill keep y'all posted