Okay I've done something incredibly stupid, I was changing permissions for admins in the Security section.

Long story short, I unticked full administrative rights for admin and pressed save. D'oh!!

Now I can't access the Admin section at all. Any idea how I can reinstate the permissions?

I had a poke around php MyAdmin but couldn't figure it out.
AAAAARRRGGGHHHH!!!!!

UPDATE **SOLVED**
Phew!! I found a solution from Pigeon here:
http://www.silverstripe.org/general-questions/show/8446

Security::setDefaultAdmin(admin, password)

I didn't think it would work cause it was the Administrators permissions that I removed but this sets up a whole new Administrators section with full permissions and I was able to undo my mistake :)