Lets not confuse the law with the flying order book. I have just added the requirement for pink short wearing in 28 days to mine. I am not sure the law lords will support it though......
If the pilots order book was required by a statutory instrument it would be law in terms that not complying with it would be contravention of the statutory instrument that requires it (The ANO).For instance an Operations Manual is required by statutory instrument for public transport
The main reason for a pilots order book is not to make unenforcable law but to cover yourself, and your company/club in the event of a negligence claim.
When I write in our POB that no one will enter or vacate an aircraft with the engine running its not just because I think that to do so is the hallmark of a poorly run and discilpined school, it is to cover my arse if ever anyone does get injured by a propellor. I cannot turn around and say its the law but I can produce the signature of the pilot who is now missing a limb in court saying that he understood that it was against our rules to try to get out of the aircraft with the engine running and that he was in fact negligent not me--something for all you self employed instructors out there without liability insurance to think about!