From the M40 take junction 9 and follow the A34 towards Oxford. Take the 2nd exit off the A34 marked Bletchingdon and Islip B4027. Follow the sign for Bletchingdon over the bridge crossing the A34 and you will immediately see the entrance to Heathfield on your right.
From Oxford take the A34 towards Bicester. Take the 1st exit marked Bletchingdon and Islip B4027. The entrance to Heathfield is halfway up the slip road on your right.