If you are fitting a new socket with a METAL back box doesn't the box have to be earthed?

If you have to ask then you shouldn't be the one doing the work - but then you knew I would say that. I've checked in the book I have on Plumbing and wiring - some of the pictures show a terminal built onto the metal back box, most of them show a straightforward earth terminal where the other terminals are. However, I do remember that if you look closely you will see the earth terminal connects to a metal strip which connects with the screw holes connecting front to back.