External oil boiler fault

[Mojo01]

Since the stat's already been replaced and it's still happening, I'd stop looking at stats and Hive and start at the boiler end. Next time it fires on its own, look at the status lights on the front and see whether it's firing on a heating demand or a hot water demand, that narrows it down a lot. On a combi a passing hot tap or a sticky DHW flow switch will fire the boiler with nobody calling for heat, and Exedon's right about the frost protection on these too. If it's showing a heating demand with everything switched off, then you're back to the wiring, a stray live on one of the switched lives like he said. Best time to check with the multimeter is while it's actually firing, that's when you'll catch it.

Great, thank you