any ideas why my gas doesn,t go through rangemaster 110 leisure duel fuel?

I have bought a second hand rangemaster 110 duel fuel but after connection I,m not getting any gas through to the rings or ovens gas comes through the pipe and thers a small screw just above the connection at the back of the oven which I assumed was a cut off valve but when I unscrew this gas escapes but does nothing else any ideas what it could be?