worcester 25 si roughly about 8 years old ?

I presume you are checking the resistance to see if theres a fault on the gas valve. You can quite safely say that if there is 1000ohms or so the solenoids are ok, check theres voltage to the valve (do this with a hot tap open) if there is then it may be a fault with the valve internals i.e. stiking diaphram

nev
November 2009