Martyn, can you tell us exactly how you got to the circle, please? Where did you park and set off from, and which route did you take? I remember when walking up over Great Staple Tor to Roos Tor, once, that there's a flagpole on the latter, warning walkers if 'Live Firing' is going on, as the Army still uses parts of Dartmoor for their training exercises. Langstone Moor seems to be in the centre of a 'Danger Area' in red on OS map. Even going to Belstone Common's Nine Stones earlier in week I had to check the board for safe times to walk over the common.
(Reminds me!... On the way to Devizes in July I stopped in a layby not far south of the town for picnic lunch and was shocked by sudden booms of live firing from Salisbury Plain!)
Something is not right. This message is just to keep things from messing up down the road