Ok I found the rear heated screen connector there is a small plastic cover to right of the side covering above the parcel shelf as you look when sitting on the back seat facing rearwards. Its a pain to get to so I ended up removing most of the plastic trim.

I disconnected the feed wire yellow/green (I think) and with the rear heated screen switched on and a multimeter set to 20v one probe connected to the feed wire the other earthed on the rear door connector bolted to the rear wheel arch I got no reading.

I have followed the cabling to just under the front dash and can see no visible damage. The relay clicks and the fuse is fine, there is 12v being fed to the fuse but I have not checked the voltage to the relay.

Looks like the screen is OK but its not getting power , I suppose I need a wiring diagram or take it to the garage??