Having made a start on it now, it is not going to be too difficult to repair. The blinds are fixed to a frame which is held in by 7 or 8 screws, plus a couple that hold a filler panel in place. Once I had removed the screws the blinds frame could be taken away and the rotten frame was there to see. There are a few metal bits to remove, these are like C shapes and have a single screw which tightens them up clamping the window frame in place in the aperture. One strip of wood was crumbling away on the side of the aperture, the other side looked like it could be used but I will replace it as it had been damp at some point. The top strip is not so easy to access with the cupboard in place but it is ok. This bit has different clamps fitted which are screwed to the wood first and the window frame is hooked on. The bottom strip is rotten at one end so I may just cut that end off and replace it - it depends on how easy it is to pull the wood off, it is fixed in place with a very strong body panel adhesive, something like Tigerseal. I will continue with the job on Monday after my mechanic has replaced a couple of brake pipes and got the MOT done.
If anyone is interested I can post a video, can video be posted here? If not I'll put it on YouTube.