This Head Race has a website.
There is a Google map for this event.
This a new note for supporters written in 2007. Please see the event's website for details about parking at Henley.
It takes just under two hours to get to Henley from Bath. The journey time is about the same whether you turn off at Reading and go through Wargrave, or go on to the Maidenhead junction.
Parking for most events at Henley is in Butler's Field. Which is marked on the location map.
There is plenty of room for spectators and good views of the race from the tow path.
If river conditions are bad, the event may be transferred to Dorney Lake. For details of the venue, have a look at the entry for Wallingford Regatta