Whenever I take the track up this side of the river, this place has stood out to me - it's just a little bit different from everything around it: just the spot that, in a moment of fantastical whimsy, you might imagine to be a doorway to some fey realm.
The only denizens that I encountered there were some frogs, and I can warn you now that an incautious approach might leave you with wet feet!
The cache itself should be easy enough to find, it's a small screw-top container, enough space for a few small items. I forgot to place a pencil there - I'll go back and do so at some point, but I advise you bring your own pen!
And please, treat the "Old Man" gently - you needn't monkey about too much. :-)
New information: It seems that heavy rainfall turns my idyllic brook into a waterlogged, fly-infested swamp! No obstacle to the intrepid cacher, but consider yourselves warned... :-) First to find kudos go to Tankengine. Congratulations!