This should only be attempted by a C.O.R.G.I. Registered Engineer....!
You need a U-Gauge... First off check for adequate pressure at P1 as per the gas regs, then connect the gauge at P2, energise the valve and see what the reading it.
Pointless doing this though unless you've checked whether or not you have voltage at the solenoid in the first place, use an electric meter on AC Voltage and check for 220V->240V AC on the solenoid.
GasAid.co.uk, December 2006
