Have just spent all morning packing and repacking my camera bag.
It is a small rucksack and will fit all dimenstiosn except the 16cm, it will fit at a squeeze. It currently sits on the table in a relaxed state at 16.5-17cm but I can squash it to 16cm and feel confident that it will fit, but you never know! I have tried everything, velco on the internal pockets and external straps.
Anyone with any first hand experiance of whether this will be ok at LHR T3. I am travelling with United on Thursday.