Collins Bird Guide, by Lars Svensson, is one of the best books for ids. All illustrations rather than photos.
Thanks for the feedback.
My 75-300 is at times shocking, CA is a big issue on some subjects and it is at its sharpest at 150mm or so.
I've rented a 70-200 F4L for this weekend, I'm thinking of buying one and using it with extension tubes for butterflies, dragonflies etc and without as a walkabout lens. Hopefully the optics will be better.
Edit: Minimum focusing distance is 1.5m or so on the 75-300 and 1.2m on the 70-200 F4L, so not a lot in it really.