Returning a Hertz car at Heathrow. What's the best way to get from there to London? Train, cab, Uber? Thank you. We're staying at the Chesterfield Mayfair, 35 Charles St. Mayfair, Westminster Borough.
Depends where in London.
Heathrow Express
Depending on where you are heading, National Express bus may also be a possibility.
Piccadilly tube line cheap & direct to the very centre of London is also an option but all this is speculation until the OP tells us whereabouts in London they want to go to.
We're staying at the Chesterfield Mayfair, 35 Charles St. Mayfair, Westminster Borough.
Emma is spot on. Piccadilly line to the Green Park tube stop, then a few minutes walk. The tube ride from the airport to Green Park should be about 45 minutes.