Actually, having just checked my downloaded mod files against Jquery Zoom files, I can see that the only files interfering with this installation are files from Zen Lightbox 1.5. I can only assume that I'd need to remove this mod and use Jquery Lightbox instead?

I thought it was interfering with IH2 mod because clicking my thumbnails was opening thumbnail sized images in a new tab as opposed to full sized images in a new tab. However, I've compared IH2 install files to JQZ files and they don't appear to clash at all?

Here are the only three files that prompted for an overwrite:

/www/includes/templates/CUSTOM/templates/tpl_modules_main_product_image.php

/www/includes/functions/extra_functions/zen_lightbox.php

/www/includes/modules/CUSTOM/additional_images.php

All three files are from Zen Lightbox 1.5..... So rather than merging files, would it be easier to just uninstall Zen Lightbox 1.5 and replace it with Jquery Lightbox and JQZ ?