Term reference field won't display
It turns out that my theme was hiding any term reference fields by default, which I discovered after switching back to the default theme (thank you too @NoSssweat for suggesting I do this).

I created the theme using Artisteer (please keep comments about my decision to do so to a minimum - I only use it as a starting point) and following @JimmyKo's comment I found the following lines in the node.tpl.php file:

$terms = themename_art_terms_D7($content);
hide($content[$terms['#field_name']]);


After I commented these out the term reference field started appearing. Thinking back, I may have ticked an option in Artisteer to say that I didn't want tags to appear so I can't definitely lay the blame on the software!