Mark to the rescue again - Thanks!

The glide.ui.clickthrough.popup property has done the trick by replacing the normal icon with a open in a new window/popup icon.

The only thing I noticed was in the new popup window which displays the user record, the related lists are missing at the bottom of the page (role, groups and location), compared to when you click through using the normal icon. I compared both URLs:

Normal Icon:
https://test.service-now.com/sys_user.do?sys_id=272a31af0a0a3c51006ca10144f19622&sysparm_refer=incident.do%3fsys_id%3dc47b32d40a0a780e0030d2d86bd997f9%4099%40sysparm_view%3d

Popup Icon:
https://test.service-now.com/sys_user.do?sys_id=272a31af0a0a3c51006ca10144f19622&sysparm_view=&sysparm_stack=no&sysparm_referring_url=tear_off

It looks as though the "sysparm_referring_url=tear_off" parameter in the 2nd URL is causing related lists not to show. How to I customise the Popup Icon URL to remove this parameter and allow the related lists to show?

Thanks