"Is then the difference down to atheists saying 'I require proof' and agnostics saying 'there cannot be any proof'?"

I think that misses the severe scepticism of atheism. I can't pretend that we're sort of neutral observers who would be easily swayed one way or the other given the production of proof. We ask for proof knowing full well that it can't ever be delivered. It's our way of showing how irrational religious belief really is. But it isn't wicked and nasty to ask for evidence of God. Given all the problems that religion can cause in this world, it's a perfectly reasonable request. Atheists would readily back off if organised religion backed off, but we all know that that isn't going to happen, so we just have to keep on making the case. Agnosticism, on the other hand, is fence-sitting. An agnostic will tell you that there's no way of knowing whether God exists or not, but he will fail to give you the dismally-remote odds, taking into account the lack of evidence and the blatant breach of all the laws of physics that God requires. It isn't a very honest position, and, as I said, there are probably very few anyway, most of those who declare themselves thus being simply uninterested in religion (and some may think that "don't know" equals "insurance policy...")