I imagine this current dry spell means it'll be a bumper year for crop marks, looking forward to whatever gets photographed being brought to the wider public's attention. As to your ring, well there deffo looks to be something under the surface (drip gully if it is your roundhouse? maybe a stock pen associated with the other settlement?). Don't forget, many fungi, and their underground mycelium can leave these kind of marks too. If you have a nose at Google Maps satellite view, on much grassland, esp in hilly regions you'll see 'faery rings' all over the place, some are enormous.