Last couple of weeks the following happens: Boiler goes to lock out, F 28 fault on display. Manual states no gas, insufficient gas or elecrodes problems. But after resetting the boiler about 4 times it all comes back to life and stays like that for up to 2 or 3 days. Then lock out occurs again, but I repeat the reset, again after 4 attempts or so and it is back to normal. Looks like I could do this indefinitely! On my own visual inspection - electrodes firing OK (good sparks visible), 240v on the PC board gas valve terminal, gas supply in the house OK. What is likely to be the cause, before I entrust the problem to a qualified heating engineer ?. I suspect a slightly blocked gas valve. is there a serviceable filter inside ?or is simply a case of a new valve - full stop. Any advice greatly appreciated.