In reply to pmorris433:
You will need to head up to the kenwood house end of the Heath. It's unrestricted parking on the road there.
Head up to highgate village and go down hampstead lane. After about 0.5 mile park up anywhere and the Heath is on your left. Landmarks are a school on your right (keep going past that) and you don't really want to go too far past a road called stormont in terms of parking.
Brilliant running on the Heath, pick your route! I usually head past kenwood heading west and keep trending slightly right at any junctions. I end up on parliament hill. A full loop of the main Heath would probably be 5 miles ish and pretty hilly