lakedistrict
I'm guessing it's the black area visible on the DigitalGlobe and Esri imagery. This article would suggest that it's acessed from Highdown Way but also suggests that it may not exist anymore - https://www.westsussextoday.co.uk/news/dangerous-for-children-s-play-areas-in-horsham-to-disappear-1-6595692. Needs a survey.