I copied the product_details.html and renamed it product_details_cran.html and have not yet edited the code. I uploaded the new page and then I went to the product and entered product_details_cran.html to the custom template field under 'appearance'.
 
However, when I view the sample product details page, the product details are not there, it just displays an additional header code (logo and menu) where the table should be, and doesn't display the table with the blocks.
 
Have I missed a step?

I think you can try to copy another template 'block_product_details.html' as 'product_details.html' has general CMS structure.