Ideal Elite wall-mounted gas boiler pilot light will not stay alight?

My Stelrad Group Ideal Elite model gas CH boiler pilot light starts immediately the ignition switch is pressed, but it extinguishes again as soon as I release the gas valve button, no matter how long I keep it depressed after lighting.

The user instructions say keep it depressed for 20 seconds after lighting....I have evn kept it depressed for over two minutes...and it still goes out immediately I release the valve button....and now I am depressed instead!

Sounds like it must be a faulty flame sensor (electrode?) but I could seriously use some advice on this daft but obviously important issue!