Some good pictures of it were posted here back in 2014. (I'm pretty sure the member now shown as "3601en9py6a5m" who posted just above me and to who I was teplying, was Matt Emmett, the photographer in the article you posted Maddy. It seems like he was just discovering the reservoir at that stage.
It was built after the Hornsey Wood Tavern was demolished. I marked the site of the tavern on Google Maps some years back. It is on the south-western edge of where the reservoir is under the ground. The reservoir is under the large flat area, to the north-east if the marker.
Remember back when I was Park Keeper. When it was opened up from inspection.
Could be used used storm water . To far gone for Drinking water