> what fascinates me is the idea that it is the ditch that is the important aspect, as opposed to
> the bank

I've pondered on the significance of the internal ditch many a time. Keeping spirits in is the best I can come up with!

Don't forget though that many (all?) irish henges have no ditch, also that some of the earliest stone circles could have been built instead of henges due to the lesser effort required. I can't recall which two Burl compares but he looks at a henge & a circle both of which had the same sized internal space, the circle however required (I think),less than half the effort...
There is also a suggestion of banking at Castlerigg and other cumbrian sites.