This cache is very close to Day's Lock on the River Thames, close to the village of Little Wittenham. If you are walking along the Thames Path you will need to cross the river and weir here. A pound lock was built here in 1789. The lock house (built in 1928) is not next the the lock, but a bit further south next to Little Wittenham Bridge, which is where the annual Poohsticks Championships are held.
You can park opposite Little Wittenham church, only 5 or 10 minutes walk away. However I would recommend parking in the car park just south of Dorchester Abbey and walking from that direction. You can do a lovely circular (or rectangular!) walk around the area occupied by the pre-Roman town, which was protected by the Thames on the west and south sides, by the River Thame to the east and by the "Dyke Hills" earthworks to the north.
You are looking for a medium sized container with space for swaps and trackables. Please rehide well.