Quantcast, the providers of the privacy tool (which creates the pop-up) have come back to us. It turns out that the latest version (35) of the Quantcast CMP (released at the end of Sept) does not work on older browsers. IT have switched all our sites to version 34 of the CMP and users on older browsers can, hopefully, now close the CMP modal window and browse the site.
I'll be interested to see if this improves matters for those affected.