Hello Doktor and well done. Hope we had plenty of water with you.
I managed to get there about 2 years ago, I can't remember exactly where we set off from without a map in front of me to jog my memory but I can confirm that the bog is a particularly nasty one and we had to detour up on to higher ground to reach it. From memory this took a good couple of hours. I've got some photos somewhere that I took. I remember that one of the stones appeared to take on a "quartzy glow" in my piccie which wasn't evident when inside the the circle - probably a duff roll of film.
Like you we appreciated the isolation of the place but were bemused to see a mountain biker chugging over the hills and across the site.
Presumably you spotted the big standing stone nearby?
Dartmoor can be a bit scary when the weather turns - even for the experienced walker. I posted up a warning about Conies Moor stone row on Friday after trying to get their two weekends ago. Langstone Circle is protected by a fearsome bog too!