Ok - I have just figured it out after tinkering with my browser settings;
The issue was an ad-blocker which uses cosmetic filtering/element hiding to remove DOM elements (including the "like" button). If anyone uses uBlock Origin, Ad-Blocker Pro, or any add-ons which feature such filters, it must be turned off in order to see the "like" button.
Pegasus - I'm not sure if you want to delete the thread or keep this up in case anyone else has the same issue that I did. Approx. 27% of internet users have installed an ad-blocker so I may not be the only one.