Thanks for the information on here, I've been able to change my barn doors to a tailgate with a window. I was lucky enough to get the same colour as well - what a result. I wouldn't have attempted it without reading this thread though!
Here's a few tips for anyone trying it themselves.
1. - If you can, get someone to give you a hand.
2. - Make sure that you have all the replacement parts before you start. ie catch plate, thread plate for hinges, struts and rubbers.
3. - If you get the tailgate from a scrap yard, make sure you get the thread plates for the hinges out - use tin snips and a crowbar if they don't have cutting equipment (they are sealed in place inside the bulkhead).
4. - Put the hinges on first. Gaffer tape the thread plates for the hinges to a stick and feed them into the baulkhead, screw in one bolt to hold them (once they're lined up), then you can use one bolt to attach the hinge at an angle. You can remove the first bolt and line the others up.
5. - Take the barn doors off etc. and replace with the tailgate.
6. - If you are lucky, the tailgate mechanism would come with a key. You will need this to remove the barrel as I don't think there is any other way to get it out. You can then replace it with the barrel from your own barn door handle.
I still have a few things to sort out like the rear demister, central locking, washer and wiper etc. ( any tips? )