From M5 - A38 to Totnes. Exit - Totnes - to Dartmouth (A379) - on approaching Dartmouth take the Townstal Road turning opposite the gates to The Royal Navel College. Take the first left onto Mount Boone. Mouthaven is half way down on the right.
Note: Please note that if you are using a SatNav we would advise you not to turn it on until you have passed Totnes as it will try to send you via Torquay and the ferry