There's no direct service.
On Saturdays you can take service 758 from Stop 2 of the Greenline Bus Station (next door to Victoria Coach Station) to Hemel Hempstead. Then take the 500 from Stop 4. Both services are operated by Arriva (The Shires & Essex). The total journey time is 2 hours 10 minutes.
Service 758 doesn't run on Sundays. Take the X90 Oxford Express (operated by Oxford Bus Company) from Bay 11 at Victoria Coach Station to Sandhills. Cross the road and take the 280, operated by Arriva (Shires & Essex). The total journey time is between 2 hours 10 minutes and 2 hours 20 minutes. (This route is also possible on Saturdays).
Bus information is here:
http://www.travelinesoutheast.org.uk
Your alternative is to take the direct train from Marylebone. The journey time is just over an hour. The fare is �10.80 single or �17.30 for Saver Return.
Chris