Thanks Rob for the measurements. Thanks Bruce for the picture of fitting point.
First the confession/excuses: 1. I have been hanging around Morris Minors too long (since birth), and 2. our rotten weather has discouraged me from venturing out through the driving winter rain and mud to my unheated shed... However I can now report that the bumper fitting points are NOTHING like MM and my confusion was caused by what are probably non-original bolts and washers inserted from behind the mudguard. There is still something interesting going on with the washers, and no original spacers. Solution is simple though - longer bolts ! Phew.