Are there different fitments for UK gas cookers?

I am trying to fit my gas cooker but the fitment doesn’t seem to fit in the socket. I have asked my friend to have a look at it as he is a plumber and he can’t do it either. He doesn’t seem to know if there are different fitments though.
Help!!

If it’s a bayonet type fitting, the standard one is 1/2 " bsp.
You can also get a 3/8" bsp bayonet fitting and you will need the right hose for the particular fitting you have got.