My Ideal Response 80 Combi boiler's gas valve keeps sticking (or at least I
think that's the problem)

I've been through the fault finding in the handbook and all the lights,
voltages and sparks appear correct at the right time for ignition, but the
boiler does not light; currently 19 times out of 20. :-(

This has happened twice before but it has fixed itself and the boiler has
then worked flawlessly for months before the fault eventually re-occurs.

When it fails to light there is little or no burner pressure so I suspect
the gas valve is sticking. The solenoids both have some resistance (4000
and 800 ohms) so I don't think it's them as the fault is intermittent. Do
gas valves fail in this manner?

Is it a simple case of a new gas valve? If so where's the cheapest place
for a replacement? Just googled and found one 163+ VAT + fitting. Ouch!

If this is a common problem is there simple remedy (such as a sharp tap on
the 'girdle spring' or whatever) to free the internal mechanism. Any of you
experts know?

Cheers

Neil