Many thanks. That was extremely helpful. I have also had a chance to learn something (ATAN2) about maths that I was was not taught 50 years ago. Sadly, whilst attempting to solve it myself, I ran into "quaternion" maths; of which I understand 0% and don't have a clue to use; but I presume may be even more satisfactory to use.

However, markschum's solution is good enough for my purposes. I am also grateful to Englishbob; who reminded me that I really should learn a bit about from the manual about joint editing.

Thanks again.