F1 fault code on morco fed24ed3

[Illuminate]

Morco feb 24ed3 has a f1 fault code. The burner reading is fine. The fault is intermittent, sometimes the boiler won't ignite on either hot water or heating. Everything seems to run but it just fails to ignite. I've changed the rectification and the spark electrodes. Cleaned the burner and the injectors and the burner pressure seems fine. I've also cleaned the venturi and tested that the spark generator is getting the correct voltage at j25 on the pcb.

Any help welcome?