Turn left when you reach the road, along to the Hen and Chickens, but don't go there, unless you're thirsty, but turn right then take the footpath on your left.
There's more details about this series (including parking) on the Chiltern Hundred web site.
You're looking for a cylinder about twice as long as a 35mm, and with the same diameter. Remember to collect the letter value in the log.
Unatvat va gurer.