A series of caches that follows The Hertfordshire Way leg 13 (from Bishop's Stortford to Hare Street), For further information about the Hertfordshire Way see link here
The caches should be easy to find, and are all on or near the footpath.
Although not strictly part of the Hertfordshire Way we thought it was only a 20m or so detour. As long as you stay on this side of the bridge you'll be safe!
You are looking for a 35ml film canister.