Final post.
The solution I described above was less than perfect, however, repeated attempts to load the new fonts into GIMP failed. I trawled the internet in search of advice, nothing I found worked.
Giving it one last try today I uninstalled both GIMP and the GTK Runtime Environment. Downloaded the latest versions of both, installed, and hey presto! All the new fonts are available.
I did try this once before, but I uninstalled GIMP only. Perhaps taking out both programs did the trick - ?