[waikiyi]

Thanks for that!
After scanning the internet I thought I'd try the 'tear the panel off' method - easy does it! Not only did I discover what was catching, but found a way to fix it until the spring rusts away. Not anytime soon!
I had to cut a hole into the plastic sheet, but that was easily taped up once I finished.

Wind the window up for protection.
Inside, if you can find how to unlock the door again, you will see a white piece of plastic with a ring. In my case, if that dropped (or was pushed down) the door jammed. If I pulled it up, voila, the door opened. Fiddle around with a wire to figure out if your door's the same.

I went to the hardware store, bought a few (incase I broke some) small springs (approx 5 cm in length) and stretched two of them just short of the distance it needed to go. I left the centre tightly coiled and only pulled the edges away. I hooked one end of the spring into the ring of the white plastic thing and pulled it up until I could hook the other end into a hole higher up, making sure it didn't block anything else. Gotta put some effort into this part.

Note: Don't buy a spring that's too thick, it could brake you're plastic bit, and might be impossible to stretch. Mine has a diameter no bigger than a pencil. Don't overstretch the spring; you'll need it to spring back.

After reading about other people's doors, I decided to do this to both doors, even though only one was jamming and they've both been perfect since.

Hope this isn't complete jibberish. A bit difficult to explain if you don't know what the heck you're talking about.