Ferroli Tempra 12 will not run?

This boiler has been problematical from the outset. It has been fitted with a new PCB and gas valve module. It has done very little service. It was running fine then stopped.
The general condition internally is very clean.
Stat and programmer are calling for the boiler to start.
Water pump is free and running.
Water pressure is fine.
Sparking is fine.
Gas pressure upstream is fine.
Gas pressure downstream (at reset) - NONE (measured at gas valve)
I have substituted the PCB with a new one (provided by Ferroli FOC following the last lot of trouble) - no change.
What please is the most likely cause for there being no gas at the burners?
Any suggestions gratefully received.

John Taylor
March 2006