This is part of a geocache trail along Beeches Way, a footpath which touches the banks of a tributary running into the River Thames near Cookham and Bourne End.
You can park either in a layby next to a footpath on the east side of Ferry Lane, in Hedsor Lane, or in Cookham itself (by the pub?). All involve a little walk.
You're looking for a camo'ed container containing children's treasures.