Ok, solved it. As I'm assuming no one else has had the same problem, I'll post the solution for future info

So it was to do with Win 7 permissions, I'm sure I had the correct Administrator permissions set on my account and for the project install folder. Anyway the quickest / easiest way for me to register the DLL was:
1. Click the Start Button, then in the "Start Search" box type CMD but DO NOT press Enter yet.
2. In the list above, under "Programs" right-click on CMD that appears above and choose "Run as Administrator" and click "Continue".
3. In the black command line window, (navigate to the dll folder &) use the command “REGSVR32” to register the file again.
Which I found in this post:
http://social.technet.microsoft.com/Forums/en-US/itprovistasecurity/thread/71037d62-d842-44a3-86df-6ed74df6fc39