Halstead 50/50 boiler pilot light?

I have a 50/50 Halstead boiler, when the system lights it runs as normal for a couple of cycles and then whilst the system is not demanding heat and the boiler is case is warm, the pilot light goes out because the gas valve shuts off with a click, is this due to the elctrode not being within the close proximity of the pilot flame, or the flame being too weak, (which it appears to be), either way do you know how to stop this, thank you.

Nigel Lewin
July 2008