I recently replaced a plastic light switch with a brass victorian type.
Coming from the ceiling were 2 x red wires and an earth, which
connected securely to the metal mounting box.

Question: There is an earth terminal on the back of the brass
faceplate. Do I need to connect this to the earth on the back box?

You need to connect the cable earth to the back of the brass faceplate. You
can also run a flying lead from the faceplate earth to the backbox. This
isn't usually required, but is often taken as a sign of a conscientous
installer.

Don't do it the other way, though. The circuit earth should go directly to
the accessory, not the backbox. It was only run this way before because the
existing white plastic switch won't have had an earth terminal.

Christian.