Baxi Bermuda Inset 2 50/4E lockout

I wonder if you can help.
I have an intermittent problem with a Baxi Bermuda Inset 2 50/4E back boiler.
The pizo starts and the pilot light comes on but the boiler doesn't always light. Resetting the boiler by switching odd and back on again resolves the problem - until the next time !!
Should I suspect the PCB or something else ?
Any help appreciated.
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