Apologies, I misread the question and thought you were asking about the Sigma for your current camera.
Anyway, the Sigma is a very good lens indeed - as I say, mine is an older version (the first version of the EX lens), but that is pretty close to the Canon lens in performance terms. It's not as sharp at f2.8, though, which might be an issue for you. At f4, you would be hard pushed to tell the difference. The Canon lens also has IS, which might be an advantage, although it doesn't freeze subject movement, so for your requirements, not that big an advantage. For me, the Sigma is ideal, as f2.8 is not required all the time, but for you, that extra sharpness may be worth paying the difference for.
Bite the bullet
It looks as if I will have to bite the bullet, many thanks for your help. 2.8 is where most of the use of the lens will come from and I believe the extra stops the IS will deliver might also be worth it.
Once again many thanks
As well as being pretty good at high ISOs the 7D also has pretty good AF - even in low light I've found it will lock on and track most things (it may not be as good with lenses slower than f/2.8 though).
If you're planning on shooting wide open a lot of the time there's probably not going to be much room for focus errors. Is the AF on the Sigma lens as good as the Canon?
All ordered, it is a 7d with the Canon lens so it is more can I keep up with the camera than the other way around. I just hope it can deliver at nearly three times my original budget.
Thanks all for your advice!
The 7D has a smaller sensor so the same lens that you'd intent to use on a 5D mkII will be a longer equivalent - a benefit to your far away subjects anyway. Not to mention the price difference too.