Potterton H24, goes to lockout after stat turned down

[Ted808]

No paperwork - littl' ol' lady with no idea-
She turns the roomstat down at night and then in the morning when she turns it up, boiler doesn't fire, she has to go an reset it, then it works fine again.
Fan is a little noisy/ 'gold' pump at boiler end is quite noisy/ gas supply is 22mB working@mtr / condensate trap, pipe clear
Of course I coudn't replicate the fault in 2hours...

Would like to change the 'gold' pump, the fan, the pcb, powerflush and....... ha ha the whole lot?
ANY IDEAS

 

[Gasmk1]

is it tripping out on overheat

 

[Ted808]

is it tripping out on overheat

Couldn't prove it...

 

[Ted808]

UPDATE: Electrodes where blown out of the ceramic.

 

[Harvest Fields]

Thanks for the update