I fail to see why the addition of just one extra radiator is creating extra water expansion equivalent to 1bar extra pressure (It's a good job you didn't fit 3 extra radiators)!
It is more than likely the system got overpressurised in the first place. Bleed some water off via a rad. vent screw and get the pressure back to 1.8-2.0 bar.
Are you sure the filling tube valve is not letting mains pressure build up in the system?
Aquarious
November 2009