On a trip through the area to get the other cache, I was so impressed with the Fen I felt the need to place my 50th cache.
In the middle of the Fen, there is a very large tree on one of the many bends in the river. I highly suggest doing this run in the spring when the water is higher or after a few days of rain. I doubt the river is passable any other time of the year. Ideally, you would go soon after the annual Jock River Race as they clear the river of debris and ensure it is passable.
From the put at Munster Rd the first take out at Joy's Rd is just short of 10km. If you take out in town it is closer to 13km.
Be sure to stop for Jock River Pioneers. My log from that cache has useful information about the journey.
Bring lots of water and bug spray as you will be out on the water for a few hours at least. A camera is also recommended. Enjoy the paddle!
Cache is not accessible by land. Area surrounded by private property and the area is very marshy. I wouldn't recommend a winter trek either as the Jock is an unpredictable river so ice conditions will vary. There are also not very many options to enter/leave other than the posted parking so it is very isolated. Canoe or kayak are really the only options.