I have now added a canon x2 teleconverter to my macro setup to try to take x10 magnification shots. Here is the new setup below and a test image.
This is the camera with the cream coloured teleconverter in between the camera and the MP-E lens.
This is the teleconverter,
Here is my test subject stuck in a blob of blue tac on a paint tin, ready to be photographed.
Here is a proper photo of the pin, for scale.
This is one of the 1000 focus stacking "slices" that I took as the microscope block gradually moved the camera towards the subject.
The slices were taken on 100 ISO at 1/60th second exposure, with two flashes (undiffused, one a couple of inches away on the left and one a foot way on the right.)
This is the final image, focus stacked using Helicon Focus software.
I am pleased with this result. I don't think many people are working with the combination of the MP-E and the 2x teleconverter, so it's great to see that it works. I'm going to do a few more tests and also try to germinate some fern spores, if I can find some.