Thanks Kevin,
Indeed the KPP .dic file is just a long list of words. Interestingly the .aff file appears to be a 'slightly' earlier version of the one you ship.
For the root word 'attain', the KPP dictionary has 21 words, your GB dic has 7. Just 1 of the 21 words raises a spellcheck error - attainablenesses
Definitely not a case of bigger is better, the KPP .dic file is almost 10 times the size of the one you're shipping
I'll allocate some time over coming weeks to spell checking some 'finished' books with 'your' dictionaries before I do anything further - providing it gets a cooler, was 47°C (120°F) in some parts of Sydney today, it's still 31°C at 8:45 pm.

- a plugin that will take a user dictionary and fold it into existing .dic and .aff files.
Thanks for putting me on the right track.
BR