How can I fix my knobs on my hob (they leak gas)?

Hi,

I've bought a static caravan and I've just connected the gas and noticed that the knobs on the oven seem to be leaking gas. Oddly enough, if you lift the knobs the gas rushes out and they don't seem to operate correctly anyway. Two of the knobs depress as you would expect and turn clockwise to release more gas and when returned to the off position they pop back up again. The two faulty switches do not pop back up and as I’ve mentioned seem to leak gas and more so if you try to pull them up.... odd huh?

Kind Regards,

Will

(I have of course disconnected the gas awaiting a response)