One of the mills taking part in this year's Mills' Weekend, 9th and 10th May:
http://kentwindmills.homestead.com/
Best call and check first though.
I think Sunday PM:
http://kentwindmills.homestead.com/Open_times.html
National Mills' Weekend Sunday : 2.00 to 4.00 pm
http://www.nationalmillsweekend.co.uk/nmw.htm
http://www.nationalmillsweekend.co.uk/windmills.htm
http://www.nationalmillsweekend.co.uk/watermills.htm