Well my new trade plate holders have arrived today and these have the spring clips that fit over the ends of your existing reg plate, BUT display the trade plate below the existing reg, so as to not cover it up. I have to say it's really simple but well made and they fit everything front and rear that I have in stock at the moment. I've also purchased two security wires that dog clip to the trade plates that you can run through a grill or trap under the boot/bonnet as an extra bit of piece of mind so that they won't fly off...
Plate holder:
http://www.ebay.co.uk/itm/322441729606?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Bunjy thingy:
http://www.ebay.co.uk/itm/381777653738?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT