If you have FM Advanced, open the application, then the Script Debugger, and then your database.
In the upper right of the Script Debugger window are two icons. The far right icon opens the Data Viewer. The other button "toggles" the Disable/Enable Script Triggers.
If you do not have FMPAdvanced, please let us know.
In the 20 minutes of sheer unbridled panic I went through since posting this, I have managed to unpick my nightmare.
I was able to get to the my new User form by logging in with a full access account and then using the manage security to unlock the crazy login script.
One thing that would be good is to forbid a script trigger to use [Full Access] Privilege sets (ie. make it not possible to include [Full Access] in the Get (AccountPrivilegeSetName) - this would avoid any nutty scripts you might write to script yourself to hell!)
Hope this helps anyone out there.
Thanks all - yes I must buy FM adavnced - after this nightmare I need it!