Thank you. A sensible suggestion. I've ordered one and I might combine it with a 12v fan.

As another suggestion, I saw an estate in a multi storey car park with an extension bar in the tailgate lock, but it was a fairly long one which gave a pretty large opening, so the dog could have escaped.  However the owner had put a dog grille just inside the rear tailgate which prevented that.  Of course there might then have been the possibility of a criminal unscrewing the dog grille and squeezing inside.  Except the dog wasn't a little westie, it was a rottweiler!

 

In case you're wondering how the dog was taken out, I wondered about that also, but noticed the rear seats were down and another dog grill was just behind the drivers seat.  The rear passenger doors could presumably have been used to decant little fido.