After some simple maths it looks like 7D cropped to 7MP would require roughly 230-700mm to achieve the same field of view, but with much more useful high ISO performance, and also useful for other things, and you can video these creatures as well. (£1.25k)
100-400mm f/4-5.6 L IS USM will get a lot done (or 300mm f/4 L IS) - up to £1k.
Now lets throw in 1.4x (7D might still af) - 0.2k and we get decent 600mm f/8 IS
OR 2x and it is now 800mm (MF but with IS). The centre should be still decent, I am sure many people here had used it.
Grand total ~ £2.5k vs £4.5.
I'd go the 7D route had I got the patience, time and money for this.