Details of this geocache trail can be found at the first cache.
You are walking along the River Ravensbourne within Beckenham Place Park, South-East London's biggest park. The portion of the park this side of the railway tracks is the smallest part (and home to the weekly Beckenham Place parkrun event, so the area will be busy Saturday 9-10am). There is a vast area to explore (and more caches) on the other side of the railway line.
You are looking for a large tube.
This series is being made open to all again (rather than premium only) - but caches will be archived as they get damaged or go missing, and the series will eventually be retired. Please look after the caches to preserve them as long as possible, and happy hunting!