To do the rebating I used a straight router bit slightly wider than the rebate width and then clamped a piece of straight stock onto the router table top to act as a guide for the router. Of course I had to move the guide four times to do all four sides of the aperture but that was simple enough - just a matter of measuring the offset correctly and clamping it in place.
This picture shows the router table top on top of the B&D Workmate with the aperture rebated.
This is pretty much in what will be its final working position on the B&D Workmate.
The next stage will be drilling the Router Insert Plate so that the router, with its base-plate removed, can be mounted on it.