There's more details about this series (including parking) on the Chiltern Hundred web site.
You're looking for a 330 ml screw-top beaker. Remember to collect the letter value in the log.
I stood here for a while, watching the coords wander around between Northing 43.369 and 43.374, and Westing 38.781 and 38.801. I don't know why they're so wobbly here, there's a few trees, but not many.
Here's some help. The cache is on the opposite side of the main road from the nearby house. There's a very strong cacher's trail where the vegetation has been trampled well down over quite a wide area.