Canon have a gap in their range there dont they?
I had to decide on a long zoom and the 100-400 was an option. Instead I opted for a Sigma 100-300 f4. I couldn't stretch to my ideal 120-300 f2.8.
However, the 100-300 is great. Constant f4, nice and sharp, and not overly heavy (though it is substantial).
I use a Canon 1.4x extender to take it to a 420mm 5.6 and quality is still darned good.
A quick warning though - the first Sigma I got would not focus consistently. I returned it to Jessops and got a second version which is top notch. Buy from somewhere that you can take it back easily. I think I paid £640 from Jessops using some voucher code action and "collect in store".