Baxi combi 105e turned off to stop a leak but how to turn off gas?

i want to make sure my boiler is safe until i track down a plumber. I turned off the boiler at the mains which seems to have stopped the leak. In the baxi 105e manual it says to turn off gas as well but it's not obvious which valve I should turn.

Can anyone point me in the direction of a picture or link to a video so I can double check I am doing the right thing?

The problem started last night when the boiler was on and I came down to find a pool of water and the pressure at nearly 3.5 (it should not go over 2.5).

Any help would be gratefully received!