Another vote for the "bigma" … I found mine at a UK retailer for £350 (GBP) a couple of years ago and it's been faultless.
I've skimmed the previous postings but couldn't see reference to which camera the OP is using … if it's a 16Mpixel or less, an upgrade to 24Mpixel would probably be advantageous.
Just remember, once your subject distance gets out towards the 100m range and beyond, heat haze and the like can start to affect the image, so, in my opinion, a 500mm on a 24Mpixel camera is about as good as it needs to be under many circumstances.
There's a lot to be said for simply getting closer, even if it does mean crossing the river
Good luck