How to repair sticking gas value Heatline Vizo 24?

I am getting a flashing LED to advise gas failure.

No hot water or central heating. The boiler starts to fire up, can see the pilot light for a moment, then shuts down, will then try to fire up again, then shuts down.

Reasonable sure the problem is with the gas valve. Gas supply from mains is fine and pressure is fine.

Does anyone know how I can repaire/replace the sticking gas value easily?

Merlin
May 2010