With the aid of QGIS I was able to georeference the plans for the visitor centre and overlay them on an OSGB layer:
Then, using Google Earth's terrain model and a similarly overlaid plan I was able to determine (roughly) the alignment from the Observation Point to the horizon and experiment with viewing sunrise on various dates in early April and September (previously guessed at being the right sort of time for a just-a-bit North of East sunrise).
This crude approach indicates that the appropriate sunrise dates are probably going to be the 6th April and 6th September.
Here's Google Earth's view of it - however note that this is from 400' altitude (to get the far horizon), the actual view will be through the gap in Fargo Plantation at the west end of the Cursus on the nearer ridgeline, aligned to the north of barrow 56:
To do this with more accuracy, I really need to get out in the field with a GPS, camera and compass - but this'll do for now.