How can i re-install mountain lion?

I just installed Yosemite on my old iMac (27", 2009), but I need to return to the OS I was using before, which was Mountain Lion. This is because I am a fan of iPhoto '09 which does not run under Yosemite.

In order to achieve the downgrade I started by doing a clean reinstall of Snow Leopard, this being the latest OS for which I have the disk. I then applied all the available updates and was planning to reinstall Mountain Lion from the App Store as I did originally. However, while Mountain Lion appears in my App Store account as available for download, when I press the Download button nothing happens.

I cannot work out how to get back to the functionality I had before I installed Yosemite. I would be perfectly happy to instal Mavericks, assuming it works with iPhoto '09 (which I believe it does) but I cannot see a way of doing this.

If anyone knows how to re-install Mountain Lion I'd much appreciate it. I am stuck now with Snow Leopard, which is a bore because my other computers are running Mountain Lion. Obviously I want to keep things consistent.

Many thanks