Thanks to Meursault and others who had solved this unaided. I think it is unfair, but I suppose it can’t be totally unreasonable if some people have cracked it. I have got it now.
I don’t think any posts have mentioned the significance of the word “Finally” in the preamble. It is not to be taken in the normal sense of the word, but rather in a literal sense.
A happy Christmas to all.