The NDB at Heathrow is at the north side of the airport. It's located near the entrance road and the fire brigade garage.
We drove around for half an hour searching for a place to park, after that we decided to look for a place outside the airport.
At the north of the airport runs the A4. In the streets north of the A4 it's possible to find a parking space, not far from the entrance of the airport.
Walk back to this entrance - opposite a hotel - and try to walk towards the high fence around the airport.
To the left is the entrance of the tunnel to the terminals, to the right is the building of the fire brigade.
Standing with your back to the south wall of the fire brigade building you will see the NDB in front of you.