Thanks for all your feedback.
Think I've decided to go with the Samyang 14mm f/2.8 as I'm trying to keep it on a budget. Not having auto focus is no biggie as it will mainly be used at night anyway.
After looking at some images taken with the 600D and Samyang 14mm f/2.8 on
http://www.pixel-peeper.com/ it looks like it will do the job nicely.