7 -I think iron age- burial mounds and soilmarks for further 5; originally a mound cemetery with 12 mounds alligned to two paralell lines. I could not identify soilmarks for the enclosure so far, where we should to suspect the settlement.
it is Robin Hood's Ball, Causewayed Enclosure (GEC layer site) and visible through the whole year. - check GE 10/21/2003 imagery for great display of structure's height data by low angle sunlight.
I think you mean, cropmarks are best visible by spring, when sprouting (agricultural) vegetation-cover can give best contrast of the soil-situation below. - it is true in general, not only for England.
by your example not only the differences of soil composition, but clearly visible elevation pattern also helps to highlight contours of this great neolithic object.