Boiler cutting out

I have a boiler that is cutting out. I don't know the model number, but it has a "ravenheat" logo on it.
It is a gas boiler, I have the system pressurised to 1 bar, and it does instant hot water. All the radiators have TRVs on them (yes, all), and there is no room stat. (Ex council house, cheap installation)
I have tried putting one valve on full to see if it is overheating and tripping out, but it still does. I have also tried altering the water temperature, with no effect.
After the CH has been on for a while, I notice the rads are cold, and the boiler fan is going, but the flame is not burning. Lowering the rad thermostat on the boiler until it clicks, then putting it back resets it, and it fires up fine. Also, clicking the time clock off and on does the trick.
Any ideas?
Thanks
Bob
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