This problem has now been solved thanks to the theme author gjr - I'm sure he won't mind me quoting his fix here.
After much testing gjr assumes that the following happened:
"1) At one time you had an unpublished "page" you were using for the home page, for testing or whatever.
2) Before changing that option back to "none" in theme options, the page was deleted.
3) Now when you view that option in the theme options it probably says no unpublished pages available or something.
4) However the theme option is still set to that deleted page, so when the theme cannot find it, it defaults to the gallery index.
Here is what you need to do to fix it:
1) Create a new page, keep it unpublished!
2) Goto theme options, select the unpublished page as your home page, click save/apply.
3) Now change that option back to "none", click save/apply
4) Your gallery should now operate as normal, and you can delete the fake page."
Great support from gjr