Neon light doesn't go off on an British General 45A 1-Gang Double Pole Cooker Switch

Check switch is wired correctly.

You have probably wired the load and feed the wrong way round, so that the neon is permanently across the supply side. Therefore the switch cannot isolate it - but it can still isolate the cooker because when the switch is OFF the circuit through it to the cooker is still broken.

You must rewire the switch and take care to connect the supply and load wires to the right terminals. These will be labelled - probably something like L in, N in, L out and N out; or L sup, N sup, L load, N load.

Don't forget to connect the earth wires properly.

That's done the trick. Thanks very much.