I have a good dash cam in the windscreen, but nothing at the rear. I cannot get a rear camera that connects to my front camera.  My idea to solve the problem was to fit a stand alone camera like this one  https://www.ebay.co.uk/itm/156577523699?_skw=dashcam&itmmeta=01JHQNF0TF174MJVJ572QWB6C7&hash=item2474bf67f3:g:p8sAAOSwvt9nWrN4&itmprp=enc%3AAQAJAAAAwHoV3kP08IDx%2BKZ9MfhVJKn5LrstUAPGQ6k9w9st8uBtbIQrsoGqn5xfFKTtaRtMIfEWB%2B6OZtlm84R6kACs2IWS1TDnpDkVmBAo9C6H96fngjLZsumzOlQXtOhWefqpciwlg%2FTZrOowMftDPhcQWdMYTjC2wMi%2FVm96DKCYZDbaK13XSL0WKYH9%2FNeAjvPJaqAhQIIEjN7%2Frrt7kT%2B5DHe2ONqwDm6JEJWpVh3BQ0b%2BGM47GKiakxbljS31oi6hEw%3D%3D|tkp%3ABk9SR9KNvPWNZQ inside the rear screen and power it from the 12 volt socket in the boot. The problem with that is that the boot socket seems to have permanent power, whereas I need power only when the car is in use.  Am I right about the socket? Is there anything I can access from the boot that has power only when the car is running?

You could add a split fuse thing (sorry I can't remember the proper name) to something like the rear wiper fuse which is only live when the ignition is on. Issue I guess with that would be running the power wire under trim pieces across the top of A, B and C pillar without fouling the curtain airbags, then down the back of the seats and into the boot. I'm sure it's been done before, if you have a go at using the forum search function, there's quite often helpful walkthrough guides usually available 

Good idea, though my car does not have a rear wiper (it was a no cost option on the hatchback, but had not been specified) the fuse should be there.  I do however want to avoid having to wire right through the car. I was hoping to run it from the 12 volt socket, but that now seems impractical, so was looking for a circuit I could tap into in the boot area. If the rear lights were lit when the car was running, they would be an obvious connection point, particularly if I could draw power from the rear lights on the boot lid.