Danny:-
Back at the house, ten minute's fiddling, the nuts were off, the whole letter-box pulled out of the door, plenty of room in the opening for clever Daddy to get hand through to the door catch... Job Done !
and very convenient for clever burglar too! You have raised alarm for a potential security loophole at Chez Chug, but clever Grandad has deduced that clever burglar would have to be double jointed with impossibly long arms...I think.