There is a good reason for my choice to go 6 point. My son is quite small and on a booster pad and tends to slide forward on the seat swab. With the crutch strap he is fully held in and the booster can not slide either.
I have resolved the fitting now. I have made use of the two rear seat rail mounts that pick up on the same points as the ones intended for harness mounting.
'Always an adventure, never a disappointment.'