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The gas regs allow the homeowner to temporarily disconnect a bayonet fitting on a cooker to allow you to clean behind it. Hence why the fitting is self sealing.
Apart from this domestic gas work should always be carried out by a gas safe registered engineer.
A very very small amount of gas will escape on disconnection. This is normal. If the smell of gas persists or if you ever smell gas in your property turn off the supply at the meter and call National Grid on 0800 111 999 immediately.
Christ, is it a good idea to fuck about with gas without any experience or expertise?