Thank you Rob! But the code does not work for my project, so I have to wait.

I have managed to restore functionality to the modal by reverting my foundation.js to version 4.3.2.
I am using a CDN version found here:
https://cdnjs.cloudflare.com/ajax/libs/foundation/4.3.2/js/foundation.min.js

This is the site I am working on: http://portland-climbing.co.uk/blog/topos/chrissy.php

As far as I can tell this has not affected the functionality of the other plugins I am using, but will keep you posted!
Rob

Get the latest code for foundation.reveal.js https://raw.githubusercontent.com/zurb/foundation/master/js/foundation/foundation.reveal.js
Its works for me. I also have added some css code to show full background:

.reveal-modal-bg {
  position: fixed; 
}