Thanks Sam and Lubin. It doesn't look like Bellever Sam. My camera was accidentally packed by my London-dwelling guests (I live just south-east of Dartmoor) so until I get it back I can't post any images.
Lubin, we crossed the road from the carpark and walked up a main pathway which then split. To the left we saw lots of people walking up an incline to open land. Ahead of us we saw a dark and quite intimidating pine forest with not a soul in sight. So we went straight on (of course). After about 20 minutes of trekking along the park we decided to cut left through the woods. This led out on a scrub of land covered in knee-high grasses. It was here that we found the circles. we could see the road that runs from Postbridge down to Princetown in front of us (if that helps).
Cheers
BB