I would say you can leave your car round my house as I live at the end of the Metropolitan Line but they are doing rail works on the Metropolitan Line on saturday.

"Buses replace trains between Aylesbury and Croxley, calling at Stoke Mandeville, Wendover, Great Missenden, Amersham, Chalfont & Latimer, Chorleywood and Rickmansworth. London Underground Metropolitan Line services will
connect with these buses to Harrow-on-the-Hill and London Baker Street."

The bus takes hours. I would say drive to Hemel or Watford, park up and get the train.