I've just logged back in to find lots more posts.Thank you for your help xwordfan.
Keepatit - you've confirmed my suspicion that some cells contain 3 letters. Nowhere in the preamble does it mention this possibility ('some cells contain two letters') and that was the query in my original post. I'm annoyed that there seems to be an error in the instructions.
Therefore... I can't rely on any of the letters I already have to help me solve the missing grid entries - do I have to cold solve each clue?
At least I have the theme lol!