p.s.
I have now run through the process again on a different page with the "Grid + Lightbox" gallery customised and installed as detailed in the webassist literature (above), however I am still getting the same error message as before..
Here is the link to the new page:
holiday-home-to-rent-in-looe.php
So it looks like this is a separate issue?..
Any help with both issues (this one and the first one above) would be much appreciated!..