Within minutes of posting a reply to my original posting saying it was fixed I had a message from a buyer about an item I didn't know had sold this morning due to the lack of an ebay email.

Sure enough, on checking my Seller Communication Preferences in my account the box for emails when items sell was unticked again. I hadn't done this - there must be a bug in ebay's system (what a surprise...) 

If you aren't getting emails it's worth checking that this has not happened to you as well.