Freelander hardback window

I have just bought a 3 door hardback and the rear 3-qtr window is loose. It looks like there are three screws holding it in that are under black metal caps on the outside. However I don't want to start trying to prize them off in case I break them and the glass at the same time. The window moves about a centimetre and so it is a risk in terms of access.

Before I start and do the wrong thing, can anyone advise me how to tighten this up?

I would expect securing screws to be on the interior of the car, otherwise anyone could prise them off and unscrew the window!