Found it! Actually it was fairly easy - just look for the rough grass covered patch / slight mound. Covered in nettles on my visit. The shape of the henge could be made out by following the line of stones sticking out of the grass. The central cist was choked with nettles and brambles. The smaller cist was full of mud. As Postie says, not a great henge, but I am glad I finally got to see it all the same. :)
Strangely, perhaps, this henge is exactly 100 meters south of the Ballymeanoch two stone row, From the stones, the only sign of this henge is the grass is slightly longer, the ground slightly rougher, it's not much to go on. Even when stood right next to the henge all you might say is what are these stones for ?
The henge is very slight, nay vague, almost not there at all.
But from what I could see, it had entrances at the north and south. The central area is occupied by a cairn with a still mostly covered cist. Apparently two other cist burials were inserted into this cairn. Four large stones can be found around the henge bank.
It is not a Super henge, but it is a super henge.
When I visited last year I failed to spot the henge. I know it looks easy to find on the map but I just couldn't see it on the ground. Perhaps the constant rain was putting me off?! One to re-visit next time I get chance to visit Kilmartin.