Worcester 240 - WAter coming out too hot - causing overheat saftefy cutoff?

Worcester 240 Combi RSF.

The Tempereature knob in the front doesnt seem to make any difference, as the water comes out practically boiling. even when knob is down to zero. Having a shower is starting to involve a ritual scream for someone to press the overheat reset switch on the boiler.

I've had corgi around to look at it, although given the age of this thing, it took him a while and a few phonecalls to let me know the problem.

He reckons the Gas Valve may be faulty, instead of opening to the correct amount of gas required, it seems only able to either open fully or close fully. Which means too much gas/heat than is required. This in turn seems to cause the water to overheat especially when using taps (as we need to run them slow to mix with cold water otherwise we burn!).

The Tempereature knob is sending the right voltage to the valve according to corgi.

I've looked at almost every article in this (damn fine) website to see if this has been explained before, but most problems seem to be caused by diverter/heat exchanger, so I'm hoping someone out there can give me a second opinion, I'd be most grateful.

Sorry about the long post!

Regards,

John