Thanks for the input Mongo.
I've been doing some research on the 500f4 P and it seems highly regarded, particularly IQ wise. Lack of AF and VR doesn't affect my birding style as I always shoot off a tripod. I've seen some tidy 500mm's go for not a lot more than a new 300mm F4 + T/C.
When I think about it, the two areas I really want critical sharpness are for birding and macro.
I've also been giving some thought to how much weight I'll be carrying, and am currently playing around with the idea of adapting a light golf buggy as I'll then be able to utilise my heavy, but quite stable, old tripod, for use with a longer lens.
Cheers
Kevin