Leave the A30 at Sourton Cross, signed for Tavistock on the A386. Follow this road, passing first the Fox & Hounds and then the Dartmoor Inn on the left.
You will then drive along a stretch of open moorland; as the road begins to drop towards the village of Mary Tavy you will see some green signs on the left hand side of the road warning of a 40 mph limit in 300 yds and then 40 mph in 200 yds; the turning for the stables is a sharp left hand turning directly between these two signs.
You will go over a cattle grid, just follow the lane until you reach the stables.
If you go past the Mary Tavy village sign you have gone too far and missed the turning.