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According to Hertfordshire County Council's public right of way data and previous tagging, the full length of St Johns Lane east of the church is a public footpath. Regardless of other access or ownership considerations, public footpaths are tagged as foot=designated - they cannot be foot=private.

Has the legal status of Great Amwell FP 35 changed?

https://osm.mathmos.net/prow/progress/herts/east-herts/great-amwell/