Automatic igniter on Mercury gascooker won't stop when lit

[Gemmabem]

Hi there, I have a Mercury gas range cooker which has an automatic electronic ignition system which to one and sometimes two of the burners won't stop igniting when the burner has lit. I've changed the igniter switch on the control knob, cleaned everything in sight and switched the terminals around on the central ignition unit but to no avail. It's driving me mad so any help gratefully received!! many thanks