I've finally managed to track down the problem!

I hadn't copied the following file

/includes/modules/pages/product_info/jscript_main.php

and put it into my corresponding new product type folder:

/includes/modules/pages/product_jewellery_info

(or rather I had, but then I hadn't uploaded it to my webserver!):

This meant that the product info page for my new jewellery product type didn't have a definition for the popupWindow javascript function, so it didn't work.

All is fine now.