I couldn't do this puzzle, and gave up.
I've criticised users of the answerbank in the past for providing a cable-car up the mountain that's all too easy for tired climbers to jump onto. So I thought it would be interesting, now that my climb is over, to see what help might have been available had I come here.
And I'm gratified, weirdly, to find that the solution is not here on a plate. Indeed, the recent Answerbank threads on Listener puzzles seem to be much more restrained than when I last looked (which was a while ago).
Not sure why I'm saying this except to admit that, as a keen Listener solver who has been a critic of the way people use this site, my opinions have changed based on new evidence. And, on this puzzle in particular, commenters pass my "is the solution given away to someone who is stuck?" test. [Please note this is only my view, and I know that there's no reason why anyone would want to pass that test just to make me happy.]