help!Bosch hob broken -do i need new one?

Sorry for dumb sounding question - my relatively newly installed bosch hob (its a 4 burner one, about 2 years old)had been disconnected during a gas check due to a small leak. One of the knobs for burner rings also turns round without being pressed down for ignition too, if you see what I mean. Is this a source of possible fault? Am confused as I thought the Bosch had a device to check of no flame would cut out gas. Shall I just go out and install new one (annoying) or is this a relatively simple repair and any idea of approx cost to repair and call out for a corgi installer? Live London. Many thanks