yes, I think, you are right, I only had 393 somimtried to use my limited knowledge to make at least something to work, even if far from perfect. i also couldnt find his valuemfor v+, hence my cheapmhack and recalculation to 5v. with your expert advice I may get the right op amp and try again. as i said before, I did get some sort of output, and at one sweet spot which I coild never replicate, my rudimentary processing sketch produced a pretty good looking distribution.
the one big thing I learned from this "failure" is that there is apparently random (at least to a novice) and then there is Random.
I also only have A level maths from 40years ago, so figuring out just how random my random was is beyond me. I am in awe of folk that can do maths or play the piano...
your excellent contribution has reignited my curiosity...I might well rebuild and report back, but dont hold your breath!