I don't suppose there is such a thing as a combined FCU/double socket to
allow me to just replace the faceplate on the first spurred box (haven't
seen anything like this in the shops so I'm guessing not)?

There are some triple sockets that have built in fusing. I wonder if a
manufacturer of these has a model with a fused output, as well? It's a very
long shot, though.

Christian.