We have been lucky to borrow the relevant photographs again and make some higher definition scans which have shown some detail of how the mini-roundabout was built.
According to the Times Obituary of Frank Blackmore he was well known for taking his team with him to test out ideas. The picture in the obituary shows an older man with glasses and wavy hair. Two of our pictures show a man much older than the other three team members wearing glasses and having wavy hair. This is most probably Frank Blackmore.
People on Twitter have referred to the Health and Safety issues. But traffic was lighter in those days, and besides Frank Blackmore was Wellington Pilot who rose to the rank of Wing Commander, so a little bit of traffic was not likely to worry him!
Other pictures show that at the junctions into the mini-roundabout there were raised blocks. Also the pre-existing traffic lights are still present and according to other photos were there until 1974 presumably to allow an easy change back to them if this experiment did not work