I looked at this thread earlier today and later picked up one of my books and thought 'I've just seen that photo' somewhere!
According to the book it was a photoshoot of the cast of 'the new edgy police series Z-Cars'. Dated 27 September 1965.
It didn't really click at first but the car's fitted with a sunroof; are they stood on the seats or did they take them out
ETA, Just realised there's an earlier post relating to it, which I missed, but photo had been lost.