It is so frustrating not knowing what the purpose of the different henges was! Why did they need so many? and whether they were built at radically different times. So many unanswered questions. But it's cool that you were able to find the remains of this one by yourselves - it is a bit of a thrill isn't it? Even though to Normal People it may seem a bit odd. But it's like you've found a fraying thread that connects you with the distant past, by careful observation, something that people might pass daily on the dogwalk and never comment on.
I was looking at the map again wondering too whether there must have once been something at the confluence itself (the romans had a fort there and there is a later medieval castle) - you'd think that if you deliberately sited the henges with regard to the enclosing rivers (which they surely must have done) then the confluence itself would be the most significant (in whatever way) spot? But maybe not.
The rivers are clearly natural boundaries - so I suppose it's not surprising I see on the Explorer map that they are still the parish? boundaries (so one parish must have a long pointy section between the rivers).
It's interesting to see the Stukeley drawing, I didn't realise that it included the little henge (not paying attention). Maybe he didn't include an entrance because it was already dilapidated and he didn't notice it?