Purchase Order  Reports -  Purchase Order Details Reports --Purchase Orders by Product should do it for you

Thanks Ken, 

Would I be able to edit this report so that I only see the total quantity purchased, instead of the breakdown?

I don't see why not - you'll just have to save it as a different file name and it'll appear in your "My Reports".

Update: Just looked at the report - all you need to do is click on the details section so it changes to yellow then in the Properties change the Visible to false.  Might be untidy but it works

Yeah that seems to be working, I've done some more tinkering with it to get it looking a bit better.