I've managed to fix the icon display issue.
All pages were throwing up 404's when trying to access any of the fontello files, or any of the "woff" files:
i.e.
....++plone++production/++unique++2018-03-12T20:03:10.995007/++plone++static/fonts/plone-fontello.ttf
...++plone++production/++unique++2018-03-12T20:03:10.995007/++plone++static/components/bootstrap/fonts/glyphicons-halflings-regular.woff
2018-03-12T20:03:10.995007 is the last compile time of the plone-legacy bundle.
I manged to fix this with the help of this bug report on Error 404 loading fontello font while upgrading to 5.1b3
Based on this clue I did:
- Goto the ZMI >> PLONE_SITE >> portal_resources
- Rename the "static" and "production" folders
- Goto the Resource Registry Control Panel and turn debug mode on and then off
This should recreate the "static" and "production" folders in portal_resources and magically the icons should finally be displayed again.
Thanks to everyone for all their help resolving this.