27mm is correct. People 'try it on' with hand luggage because they have seen others break the rules or have got it away with it themselves. I have seen oversize hand luggage, I have seen heavy stuff that took 2 grown men to get into the overhead locker, I have seen people with 4 items of hand luggage excluding their duty free and so it goes on. If the existing regs were rigorously enforced the word get round and the problem would soon end.
I disagree with this. I regularly travel around Asia from my base in Korea. Every airport I have flown out of, without exception, sensibly applies rules on sensible amounts of hand luggage.
The system doesn't grind to a halt becuase I have taken a small rollaboard AND a laptop bag, there aren't endless queues becuase we forgot to put our contact lense solution in a clear plastic bag, in fact there aren't really any queues at all... why? - Becuase there are a lot of security points, with a lot of polite, courteous and sensible security screeners who seem to keep the whole thing moving smoothly.... And this isn't at the expense of security, becuase people are always pulled aside for a bag search if anything out of the ordinary is spotted, and the clear plastic bag rule is spreading around here too.
In short it is the ineptitude and greed of BAA that causes all these problems.