Finished the job this morning.
To remove the wheel from the caravan and fit the spare took about 15mins or so. Jacking up with the trolley jack was a little awkward as it has to go under the caravan, lengthwise, and not sticking out the side. So, pumping it up took time because I could only do short strokes at a distance. Fortunately you don't have to jack it up as high as a car wheel.
We've decided to carry the spare wheel - when its back as a spare - in the caravan when towing and in the awning, under the caravan or in the back of the car when on site. There's no room inside the caravan to have it permanently in there.
I was going to remove the actual carrier but you can't just unhook it, so when the next trip is over in a couple of weeks and the annual service has been done and I tuck it up for the winter, I'll go underneath and unbolt it.
Got as much rust off the spare wheel as possible and was then going to spray it black but couldn't find the black spray paint. I had some Black Sandtex - no idea why because we've never used it - so I painted that on. Its better than rust