If you have a chance to watch this show I'd recommend it. I found it fascinating, and until I saw the hendge actually rebuilt I had no idea how mathematical in nature it was/is.
For instance, the 2 heel stones fronted by the 3 standing stones as you are approaching the hendge which was in fact within a point rectangle of 4 stones outlining its or an outer area.
My guess is that (2, 3, 4) the hendge or circle may be the 1 wherein they'd placed 89 bluestones which is yup just 1 short of 90 - so, currently I wonder if it's perhaps a plotting or abstraction of a 90 degree circle, maybe they were working off quarters a/o were into some sort of seasonal expressions. At any rate, it fits very nicely within the circle/hendge within that 4 point rectangle.
I also think it's interesting that inside the hendge were placed those 5 tall standing stone arrangements, each composed of two stones with one stone across their tops, perhaps, astronomical a/o constellations, days/months, dunno, but it sure is fascinating.
Evidently they've also just found another arrangement of bluestones - looks to me like the stonehendge riddle may yet be solved.