Solved it!
The answer is to withdraw the cabinet completely from the housing (take care, it's very heavy). To release the side panel take out three srews at the rear edge of the panel and one on top. Remove the side panel and peel back the glass fibre insulation until the fixing plate is found.
MAKE SURE THAT YOU SWITCH OFF AT THE MAINS BEFORE TAKING THE SIDE PANEL OFF.
Graham, October 2006