@ChrisBell. Thanks for the helpful response.
It's a tailgate with a full camper conversion, I was thinking of fixing the camera to the rear of the overhead cupboard at the back of the van as indicated by the red arrow on the picture below. It's well out of the way there and won't be knocked by animals or kids - we haven't got the former at the moment and I wouldn't allow kids in the van as the dirty little sods are worse than animals who at least do what you tell some most of the time.
No rear wiper but I do tend to keep my windows clean - I do however appreciate that a dirty window would degrade the picture.
How it's powered is a question which I've still to answer properly for myself. The two OEM 12V sockets on my dash are both permanently live as are the two currently installed as part of the leisure electrics. TBH I would much prefer it being switched on and off with the ignition but I'm not sure that I can be taking on the task of taking power from an 'ignition on/off' fuse (ie. I understand that to be one of those in the middle row of the fuse box in the passenger foot well) and routing the cable properly - not to mention finding where an appropriate earthing point would be. I might reconsider but at the moment I'm thinking of tapping into one of the existing 12V outlets in the leisure setup and fitting a socket in the rear as indicated by the green arrow but I might just decide to run an extension cable (Ie. plug to socket) from one of the rear (leisure 12V) sockets to the back (merging in the same place. TBH I would happily pay an auto electrician to do the job for me if he could run power from the front and route the cables properly but they seem to be very rare in my part of the world.
The ability to take a still image with the camera is good to have but IMHPO not essential as long as the camera records any incident. I can take still with my Kenwood front dash cam but never have.
I hear what you say about 4K but IMHO 1080p should be good enough , assume the lens quality is OK - how do the recorded images from your front and rear cameras compare?
Again, thanks for the response.