From M25 jn9, take A24 south, then Mill Way to Headley.  Turn right past the cricket club and NT car parks towards Box Hill.  At T-junction turn left.  After 1 mile turn left into Deans Lane.  At the end after 0.6 mile in Walton on the Hill turn right and immediately left at the lake in to Withybed Corner.  There is some parking at the pub, but you may have topark along the (rather rough) lane.

From jn8 take the A217 north for two miles, then left onto Mill Road, New Road, Mere Road then right at the lake.