Hi Bob,
I cut two lengths of steel bar and bent them into rough right angles to make my brackets. Once I had a good fit I positioned the rear bumper where I wanted it and marked out the holes where the M10 studding would pass through the body shell. With these drilled I lined up the bumper on wooden blocks and passed the studding through to the boot. I could then mark where I needed the holes in the brackets and drilled those. I then fixed the brackets directly to the chassis through the body shell. Much the same way as I did for the bracket that holds the latch pin for the boot lock.
Once fitted I screwed the M10 studding into the overriders with some thread lock. Passed the studding back through the bumper, some rubber washers to protect the body from the bumper into the car where I bolted them to the brackets. Its fairly strong as I now use the rear bumper to pull the car out of the garage

I don't have any pictures of the brackets but the following link shows the bumper fitting and at what height.
http://www.camnest.co.uk/simon/Images/RearBumper/RearBumper.html If you would like some photos of the brackets themselves I could probably take some at the weekend if you would like!