hi postman
Although I haven't been (although I will now!), I have a book with a picture of Sling, and it is definitely the one with the capstone resting on an upright. The book says it is also called Yr Hen Allor (old altar), and are the remains of a small portal dolmen. It also mentions a recumbent stone nearby which appears to belong to a collapsed chamber.

Hope that helps!

sam