If You Had A Twin, But Didn't Realise...
Family Life2 mins ago
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For more on marking an answer as the "Best Answer", please visit our FAQ.Do exactly what it says in the instructions - use a metal back box if it has to be flush fitting. I think the issue here might be more to do with fire safety in the event of a fault with the socket. A flush fitting plastic box might burn more readily, and you may not be able to see the fire because it's inside the wall.
As the work you are doing is in a bathroom, make sure you comply with Part P of the Building Regs or you may encounter problems later. Better still get a competent person to do the work, then they have to worry about Part P - not you.
http://www.odpm.gov.uk/stellent/groups/odpm_buildreg/doc uments/divisionhomepage/br0054.hcsp