is it one you added? Open the compose windows and move the cursor into the typing area press Ctrl + Shift+P to bring up the spell checker dialog and click the edit button.

You've solved my problem, thank you. Unfortunately, is wasn't one that I had added so I couldn't delete it but your solution took me to my added words where I would have been able to. I've just added my preferred spelling. Any chance you know how to get to the main dictionary so I can delete the word I'm not happy with?