Don't take this as biting at you or being sarcastic or nasty. Take it as a really helpful response.
Go to the main forum page. Click on forum search. Type in 'Hampstead Heath ' and click search. The many results will give you all the information you could possibly need. Much of it is repeated by the same users. And that's why I'm helping you with this post. Those that can help you and anybody else, are tired of posting the same info time and time again, but for it not to be used.
I found Hampstead Heath, the dark room and the fuck tree by following info I read on here when I first went and that was my first time cruising in a public space. If you can't follow the info you'll find on the forum search then understand that you are probably not the type of person to be going there for fun. Though you'll actually find the heath is for everyone.
Pre-Google, I cruised Paris, Barcelona,Amsterdam, New York, New England, and California....
It was called following your nose.... and reading a guide book