Now with screws in hand I was able to continue with the work of attaching the router insert plate to the router. This firstly involved drilling holes in the insert plate for the screws. (The router table insert plates don't come with pre-drilled holes because there is a wide variety in placement over different makes and models of router.) I attached the router's own base plate to the insert plate with sticky tape and drilled through the holes and through the insert plate. I then took off the router base plate and countersunk the holes using the suitably angled point of a 1/2in bit. (Probably not as good as a real countersink bit but good enough.)
Attaching the router to the insert plate was then straightforward and I set up the router 'table' on top of the B&D Workmate to trial fit it. Here is a picture showing the almost final set up.
Because the base of the router is slightly wider than the maximum jaw opening of the Workmate, I had to raise the router table up a little to clear the opening and let the insert plate sit flush in its aperture in the table. This picture shows the temporary measure with a couple of pieces of 25mm MDF offcut between the router table and the Workmate top.
The next step, and probably the final one for now, will be to attach a couple of pieces of 50x50mm timber to the underside of the table top either side of the router insert to raise the table clear of the Workmate top. I'll also grab a piece of 100x50mm to clamp to the table top for use as a fence. Depending how this goes I may make an adjustable system with a couple of routed through grooves to put adjustment bolts through for the fence.
And lastly for now, if I never have to go to Bunnings again it will be too soon!