Well, 18 months later, spurred on by the fact that reality seems set on stealing my plot lines, the long-awaited book has finally been released into the wild. You can get it, as an eBook or paperback, here:

In the UK:
https://www.amazon.co.uk/Brightwater-Major-Returns-Jessamine-Matheson-ebook/dp/B0872JN59T/ref=sr_1_7?dchild=1&keywords=brightwater&qid=1586887239&sr=8-7

In the US and Canada:
https://www.amazon.com/Brightwater-Major-Returns-Jessamine-Matheson-ebook/dp/B0872JN59T/ref=sr_1_2?dchild=1&keywords=brightwater&qid=1586887296&sr=8-2

In Australia and NZ:
https://www.amazon.com.au/s?k=brightwater&dc&ref=a9_asc_1

If you are typing Brightwater into the search bar, for some reason Amazon Australia separates it into two words and you have to select the all-one-word version.