Complications, complications, complications
Okay, a complication to this issue. I was laying out engine pieces this time and exactly the same thing happened to me when I hit the three key after I had finished and taken a UV snapshot.
However this time I had absolutely no history on any of the objects! So I was really annoyed and just messing around in the UV editor and came across the layout function. I thought that I'd give it a go as I had nothing to lose, it sort of crammed everything together and didn't look that good so I hit CTRL+Z and whammo my UVs are back to how I originally laid them out!?! Unfortunately as soon as I hit the three key they all mess up again. What is the secret????
Cheers, the Yeti.