Interesting to note that the CAA now make a point in CAP's realting to Mass and Balance that 'free seating' can lead to excessive or significant variations in CoG. Especially for pax loads of 50-100 allowed to all sit at the front or rear of the aircraft. This also applies to re-distribution of pax during flight. In such cases where pax are re-distributed or move themselves, cabin crew should advise the crew so that they are aware or a CoG shift. Also worthy of note that it tends to be advantagous to plax pax as far aft as is allowed to get the CoG towards the aft limits. This reduces the amouts of stab trim required and hence drag and fuel burn.