It appears that many NextGEN pages on my site with unique content were potentially not indexable because there was as a canonical URL on the page. Eg. http://japaneseartsgallery.com/nggallery/page-23/image/33/ had a canonical URL pointing back to its main gallery page.
Presumably this is bad so I turned canonical URL's off. I do note however that Google has indexed some other NextGEN sites pages with canonical URL's.
Have I done the right thing?