Disconnect the power. The cooker switch will be located near the cooker, or in a cupboard.

Move the cooker out of its space, carefully avoiding stressing the gas or electricity supply. These should be long enough to enable access, but do not attempt to disconnect them.

You should see a perforated circular panel in the rear cover. This is held in by 5 small lugs which can be snapped. Retain the piece to refit later.