Well the 100-400L is a lot of money, but on the Chester trip it never came off the camera despite the fact that I lugged a heavy bag full of gear around on a sweltering hot day, which I could have left at home. I expected the 100-400 to be a bit 'long' in some situations, but I found there was always room to move back sufficiently even with the closer animals.
It's a heavy lens, but you get used to bracing your elbows on your chest and relieving a lot of the strain of using it all day.