I've replaced the large metal plate behind the diaphragm with a smaller plate.
Visited a mower specialist to track down some brass washers and he advised that it was a common fix.
NOW my over-revving is sort of fixed, it's now searching through the rev range to find an idle, going from normal to over-revving to choking out.
Full throttle causing choking out and leaving the throttle on the starter speed is causing a "search" (up and down through rev range)
