F wrote:
We need to disconnect my father's gas cooker to ensure he can't use
it - turn gas on and forget to light it scenario.

Is it acceptable to simply disconnect the flexible hose? Is the
fitting it connects to sufficiently gas tight over time?

If its the bayonet(push-fit) type? the valve will close and stay closed.
If screw, this needs to be capped.
--
Sir Benjamin Middlethwaite