It worked without a hitch. Rooted now!
The USB jig put the phone in download mode but it won't get rid of the warning triangle unfortunately. Even after following the video tutorial carefully it's still showing the triangle. Shame, as I wanted it gone.
EDIT: Damn, I just found out that USB jigs don't work on ICS. Apart from using Triangleaway app, is there a USB jig that works on ICS please?