No more hard time is being had here - this old prison has been decomissioned for some time. But don't worry, lock up or not, you won't be going inside. You don't need to be sent up to find this cache, it's right here on the outside!
You are looking for a small screwtop container held vertically in a safe place by a mysterious invisible force. Please don't push it too far in - just far enough in that its handle is just inside and can't be seen. Make sure you put it back the right way up. It contains a waterproof log sheet and swaps. BYOP.
You are free to walk or cycle along beside the canal, but please be careful of the edge and take care with any little ones. The canal is pretty deep and can be quite fast flowing at times. There are escape ladders located every now and then along the sides. Fishing is not allowed in the canal.
Good luck and happy caching!