what i need to know is can a plastic moulded box be set flush into the wall (screwed to brick not plasterboard) like a metal box or do they all have to be surface mounted.
Moulded pattresses are designed so that the box face has the same dimensions as the socket (for surface-mounting), whereas the boxes designed for flush-mounting are designed to have their edges concealed when the socket is screwed into position.
You could use a surface pattress for flush mounting, but it would be difficult to get a neat cosmetic finish.
Metal back boxes are smaller than surface boxes to ensure that the socket/switch covers it neatly when it's plastered in. They also have adjustable lugs so that when it's plastered in you can still level the socket/switch.
There is no reason why you can't use a surface box but for a few pence you're making the job hard work for yourself.
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