Since I published the above, Ken (admin on the EP forum) gave me the heads up on an article by James J Jones.

He has some useful things to say on the subject, not least that you can’t rely on the Google keywords tool to find niches. I never really thought you could but I was struggling to find something else.

Have a look at Mr Jones’s advice:
Kindle Publishing

And for more advice and suggestions about trying to assess the demand for Kindle books in any niche, see this thread on the Experienced People forum.