I changed the permissions using chmod to 777 for all users and the captcha image still didn't display.
Is it possible that I haven't uploaded the font file in the correct way and it's not being recognised as a font on the server? When checking the font file in my hosting control panel file manager the file type is listed as other? I've double checked the font file name is correct and as I said before it works fine on my local development machine.
I've left the Font directory permissions as 777 for the time being and I recycled the application pool on the server too to clear the cache. Still no luck though. I'm really stumped with this.