There's no cutter, as all four holes are slightly different. They are round with little notches, and the notches are different between holes to ensure the right sensor holder is fitted in the right place.
I did mine with a drill and a file. It's easy because the positions and shape / size of the hole is marked on the inside of the bumper (at least on Mk1 75s it is).
The positions are marked on the inside of ZT bumper as well The genuine cutter is part number VUN100030 Drill the notch holes and then use the cutter to make the holes I was trying to avoid using a file and a rotating drill bit but will do if I need to. I haven't had the bumper painted yet just in case.