From Cambridge University Caving Club - CUCC wiki
Looks like 508 people days on expo this year.
Using the numbers in http://www.srcf.ucam.org/caving/wiki/Expo_2008/Food#Quantities, I (Jenny) would estimate that expo 2012 would eat roughly 36 trays of flapjack (or 18 batches).
Of course if there isn't time to make that much, expo could just buy more choc bars (or something else) to compensate.
The recipe is http://www.srcf.ucam.org/caving/wiki/Expo_2008/Food#Flapjack