How do I clean my Potterton 402856 gas valve?

I have a floor mounted kingfisher 2, CF50 central heating system. when the heating valve closes it makes a bubbling sound, on investigation I have discovered that this is caused by little flames flickering on the burner for a short time(3-4 secs) implying that the gas valve isn't closing cleanly.In the instruction manual it says clean the valve and seat but gives no direction as to how this should be carried out. Can anybody help?