This is going to be the hardest part I think. Because the frame is curved I am going to be pushed to do a tidy job I think. Still, it doesn't have to be pretty because only we will ever see it. As long as it is functional then the aluminium skin will hide all my wood-butchering. I already have wood cut to the correct thickness, all I have to do is shape it to fit. The whole left hand side was rotten and needs replacing. Here it is with some parts replaced.

And, here is a close-up of one of the new pieces I added.

Nothing left then but to offer the window up to the caravan to see if I had the dimensions correct. It looks good so I slapped some Sikaflex on and, with the kind help of my brother we fixed it back to the caravan with some 3 inch screws. Clamps added to the sides to make sure the bond is tight.