Freelander hardback window

I have just bought a 3 door hardback and one of the opening rear 3qtr windows is loose. Even when closed, I can move it from side to side and up and down. It means that it is a risk in terms of being prized open and you can almost get a finger in the gap. It just looks like it is loose but I don't wnat to start trying to take off the black circular caps over what I think secures it, in case I do other damage, if in fact when I do you can't actually tighten it.

Does anyone know anything about this and how I can tighten this window up?