Cannot download/install Lion on MacBook4,1

Hi,

I've just bought Lion to install over the top of 10.6.8 on an old MacBook4,1. I've redeemed the code I got from Apple and OS X Lion appeared in the App Store "Purchases" with a download button. But pressing the download button doesn't appear to do much, except asking me to sign in. Once signed in, I get a "loading" animation for a second, but then that disappears and the button still says "Download".

However, if I reboot, then "Downloading Lion (Paused)" or similar appears in the dock. I can click "resume", which does nothing. I can click "cancel", which also does nothing, unless I reboot again, after which it disappears and I'm back to square one, except I'm unable to see OS X Lion in "Purchases" unless I go into my account and unhide purchases.

I see nothing in downloads, nothing in applications.

Pint for anyone who can solve it.