I've locked myself out of my Ubuntu box!

From the "Its important to double check the documentation" department:

I've been having a bit of fun resetting up my file server at home. But today I managed to pull the right doozie of them all, and I can't find a way around it.

I have been trying to implement an init.d script I found to control Subversion. I made a few small modifications to the script so that I could run them from my own user account (dumb, I now know). Anyway, part of that meant adding my account to a new user group "subversion".

usermod -Gsubversion aidan

Now its time to do some editing. sudo emacs /blah/blah/etc

"aidan is not in the sudoers file. This incident will be reported."

Well, what I discover now is that, slightly contrary to what the original webpage had told me about -G (adds the user to a new group), it also erases the groups that the user is currently added to (for reasons that are beyond me at the minute...). And now I can't sudo to add a new user to sort the problem out?

Anyone got any suggestions?