My elderly mother tends to leave the gas cooker switched on which is bad when the flame of the cooker is sometimes extinguished. This allows explosive gas to pour into the kitchen. Is there anything we can do to switch off the gas automatically when there is too much gas? Surely this must be a common problem!

Common problem, yes - mothers

You need a gas detector with shut off valve. Make sure you search for gas detectors for natural gas or LPG (depending from what your mother uses). Sometimes people get confused with carbon monoxide issue! Not related at all. CO (carbon monoxide) is the problem AFTER gas is burned. The explosion in your mum's kitchen is the problem BEFORE the gas is being burned! Look for AMS S/2 LPG and Natural Gas Alarm. Then link a gas shut-off valve to it.

Cheers