Rightyho, the plot thickens.
I went around with my multimeter checking continuity and the ground was being a bit flaky, do I re did it and that sorted it. Assuming i had found the answer. Put it in reverse, nothing.
I had noticed previously that when opening and then shutting the tailgate it activated the camera (but no actual video), so I did that and voila, there it was working fine on the screen. Stayed there, no errors for a long as I was in reverse. Video was fine bar you can see the top of my number plate but will sort that.
So fixed, except it's not
If i take it out of reverse and go back in it defaults to just the parking sensor screen, if I get out, open then close my tailgate, up pops the bloody camera working fine.
So clearly the camera is working fine (well it could theoretically be the jolt of the tailgate that is making it work it but I seriously doubt it). There is no sensor display on my dash to suggest the van thinks the tailgate is open....
So, wtf!
EDIT: Could this be a VCDS setting? when setting up the camera there was a menu option that had something like "default view", the options were (roughly, from memory) OFD, rear camera and automatic. It was on OFD, we changed it to try and make it work but given nowhere does it mention changing this setting, we put it back to what it was when we started and just ticked the 2 options that are required. I am wondering if this needs changing. The closing of the tailgate perhaps triggers a different software sequence and view so you get the camera, but the default view is just the parking sensors?