Hi Alan - info. on 2 hrs free museum access was provided by the guys on the desk when paying landing fees. No idea if this was the official line at the time and haven't had the opportunity to return since then to find out more.
On another point and I'm probably teaching my grannie to suck eggs here, but in the interest of trying to be helpful it might be worth noting that there's no o'head join at Duxford so getting a good idea of joining procedures and joing direction in your head before you set off is well worth the effort. I normally give them a call just before leaving base to confirm circuit direction, rw in use etc. and then simply draw a penciled arrow on my Pooleys guide as a quick reminder as to my inbound track versus the airfield layout. Gives a great mental picture of where I need to position the aircraft for a downwind join.
Anyway, have a great day at Duxford and if you've the time try the Land Warfare exhibit too.
Cheers