By now I have perfected the process and the last 5 blocks went in smoothly after I had dug out the trench, and the final slot was just the size of a brick - as planned.
After a little clearing away of tools, materials and soil, with the help of another helpful neighbour, I moved the greenhouse frame into position. To my great relief the base was level and square and almost exactly the right size. Despite my best efforts at measuring, the base is a couple of centimetres (three quarters of an inch) too narrow but nothing I cannot sort! I pegged the frame down temporarily as I don't want the wind blowing it over and spoiling the work!
So before I left, I picked a whole lot more purple sprouting to take home (and gave some to my helper) donating another huge stalk to the chickens.