The timetable you need is here. Check for engineering works!
http://www.firstcapitalconnect.co.uk...edford_sun.pdf
If you are starting from anywhere on the Metropolitan line then West Hampstead is probably your best bet.
There may be a route from Harrow and Wealdstone with a change at Watford but you would have to research that for yourself.
PS: The BVWS website needs updating. Jeremy Day no longer organises Harpenden.