Easy, remove wires noting which go on which terminal. Remove broken plastic from front plate.
Remove back box and replace with new one (standard electrical back box).
Reattach wires screw front plate to back box.
You may need one of these to make reattachment of wires simple.
http://www.ebay.co.uk/itm/Philex-Cab...oAAOSwa81Z6JOQ