Thanks for all the replies.
I think I may go back to the idea of a 2 gang conversion back box. I was concerned about possible stability issues if a plastic pattress is just attached with two screws. Also I prefer the symmetry of a 2 gang. I already have one of these boxes and it's 5mm...
I am looking to replace a single socket with a double or triple one, triple would be good to make it more future-proof (it'll only be used for electronics so the 13A fuse won't be an issue).
I know you can buy special conversion boxes that screw into an existing single flush back box. But...