Thanks for the response and apologies for the late response myself (I've been really busy recently). I tried your suggested fixes, such as deactivating all other plugins, but to no avail unfortunately.
The versions I have are:
I could get the highslide working when I included movie details in a post using the [imdbltid] tags, it just didn't work when I used the < !--imdb--> tags.
Saying all of this, I've just updated to the latest plugin version and now the [imdbltid] function throws up the following error multiple times within each post:
Warning: filemtime() [function.filemtime]: stat failed for /home/content/75/9924975/html/films/wp-content/cachett1753496.search in /home/content/75/9924975/html/films/wp-content/plugins/imdb-link-transformer/class/mdb_base.class.php on line 276
I assumed it was a cache issue so I disabled the cache and everything works now, even < !--imdb--> functionality :) I'm pretty sure I'm missing something obvious but the Cache and Photo directories are set up OK with the correct permissions. It's only a smallish private site though (which is why I can't share the link unfortunately) so unless I'm missing something obvious then I think I'm just going to leave the cache disabled.
Thanks for your help JVC, and for the awesome plugin. It's great to see that you're still working on updates too :)