Looking more like a chair
I have managed to sort out my issue with this chair. I had threaded the cane too closely together from left to right, meaning that the holes between the canes were far too bunched. It would have been impossible to thread the diagonals through them.
Now, however, I have removed some and rethreaded others in a two hour rescue operation.
Lessons learnt about caning a round seat today: 1 never do it again. 2 If you forget number 1 for some reason, thread by eye, rather than counting holes so that your canes produce evenly spaced square holes.