A note for info............... I've landed at Duxford several times and on each occasion have been asked to pay landing fees on top of Museum entry charges. However, as visiting pilots we're apparently entitled to 2 hours free entry to the museum (a little known fact) and as there's no policing of how much time you do actually spend in the museum the landing fee is the only cost.
Best days to fly in are the Fridays before an air show as there's lots on the ground and lots in the air practicing before the big day so plently going on for free without the hassle of crowds etc.