I've only bloody fixed it Steve!
And you don't even need to take the door cards off! (Although I had taken both the top and bottom door cards off the drivers door before I discovered this!

)
Open the door fully and there are two bolts that attach the check strap to the door. Mine had either lost the nuts or snapped off the metal around them. So when you close the door the business end of the check strap sticks too far into the door and does nasty things to your windows!
^Passenger side^
Make sure the bolts are lined up and sticking back through the holes and simply replace/refit the nuts. They're standard M6 nuts (10mm outside, 6mm thread). I thought it would be a good idea to fit some slightly larger washers just to spread the load.
Seat wanted 47p each... But I asked nicely and got some from a local fastener company with a handful of washers for £0! You may not even need to buy new nuts. I found all 4 original ones inside the door as I was fitting the last new one!

Have a poke around the check strap with a magnetic screwdriver.
Also... I think my little Arosa has had his drivers side window smashed at some point!? What do you think?