london underground

follow this link and use the journey planner.

http://www.tfl.gov.uk/

I believe it's the Picadilly line - end point Walthamston Central. E-mail your hotel for specifics...that's what they are in business for. And ask for specific directions...which street exit do I take out of the underground at Green Park Station, turn left?, turn right?, one block?, two blocks?, turn left/right at the Starbucks? (yes they are everywhere).

Take the Piccadilly line to Green Park then follow the directions below.

These directions are from the Hotel website~~~~~~

Exit at Green Park station and turn right, then walk for approximately 5 minutes. The Hilton London Green Park hotel is located on Half Moon Street, which is the third street on the right.

Bob is right, just jump on the Picadilly Line at Heathrow and relax in your seat until you get to the Green Park station. We took the Heathrow Express and had to transfer to the tube at Paddington, and it was a pain having to lug our suitcases down the steps and back up when we got off again. Much easier to just take the tube!

Any Underground train from Heathrow goes direct to Green Park in about 50 minutes for 4 GBP.
Trains run every 5 minutes.

The line is the Piccadilly Line. Don't go for the Heathrow Express or Heathrow Connect trains which go nowhere near where you want to go very expensively.