That was my last attempt (10 mins ago). I had a load of stuff in my assembly folder that looked like it was old beta stuff. I tried gacutil to uninstall and it refused because of depencies to this: SCHEME: <OPAQUE> ID: <{7A1ADD0C-17F3-47B8-B033-A06E189C835D}> DESCRIPTION : <.NET Framework Redist Setup>
Annnyway, I then went down the heavy handed route of deleting the files from the assembly folder, and then rd-ing the folders from the command prompt, thinking that may sort it. But, it didn't. What was weird is that after deleting all these files, when I used windows explorer to look in the assembly folder again, they were all still there - or at least it thought they were.
Hmmm, either I missed some, or my setup is seriously unhappy!!