Biassi combi boiler won'r fire up?

My Biassi combi boiler suddenly stopped firing up. When heating or water is turned on, it sparks (until cutout after about 5 secs) but won't fire up. It appears there is no gas getting through to the burner.
Referring to the service manual, I have checked the modulating operator coil and the resistance is as should be, about 125 ohms. I then carried out the second check (also referred to as 'modulating operator coil check' in the manual) by measuring resistance across the 2 upper, then the 2 lower of the 5 connector pins located on the gas valve below the coil. The upper 2 were as should be, about 1250 ohms, but the lower 2, which should be about 3000 ohms give no reading at all, ie. an open circuit. So, do I need to replace the whole gas valve (about £120) or the coil (£30) or have I missed something?
Any help / advice much appreciated