Re Jeffs Posting.
The Opening times for the Ham Wall car park are:

01/10 - 28/02 0700 - 1830
01/03 - 31/05 0700 - 1930
01/06 - 30/09 0700 - 2100

The barrier has a timed automatic electric lock so there will be no lee-way involved - you will be locked in if you are late.