I cut out the section at the rear of the case to fit in the switch, I made sure that I left a tab at either end that I could bend over to attach the front plate of the switch to.
A couple of hours with a hack saw, and appart from the aching arms I had a pretty clean hole.
Bent the tabs with the pliers and slotted the switch in from the inside, attached the switch front panel to the case with some case screws and the switch to the front panel with it's original screws.
it looks pretty tidy but it needs a sand down to remove some burrs.
Stage One Complete.