I was hoping to tidy up the array of single FCU switches.
I had planned to use dual metal back boxes but was wondering
if anyone knows of a supplier of dual FCU switch plates?

You could do what I did and use a grid switch. I used a 3 module type that
is the size of a double socket (4 module would also fit a double socket
space). I used only 20A DP switches, running to an unswitched socket behind
the appliance. There is no fuse in the switch. It is safe to do this, even
on a 32A circuit, provided that the runs of 2.5mm aren't too long. This is
because overcurrent protection can be assumed to be provided by the 13A fuse
in the plug top. The 20A switch is protected because of this. If the 2.5mm
spur isn't too long, then earth loop impedence will be fine too. This is
required so that the 32A breaker can be used for short circuit protection
for the cable up to the socket.

Christian.