After a hectic few days at work, I have managed to read the book as well (probably for the first time) and thoroughly enjoyed it. The only real 'criticism' I could make is that it could have been longer and it felt a little rushed in places, but when that is the main criticism of a book then you know that it can't be bad.
As I think Julie already said, I felt that having Ben threaten the children with a knife seemed rather extreme when viewed with modern sensibilities. It was a very quick turnaround from this to having him invited to tea by the children's mother!
I don't know why, but I also found that scene particularly amusing. Having invited someone to tea, their mother says something along the lines of, "Nice to meet you, but I've got some work to do so go into the other room and don't come in here." It obviously served a purpose for the plot, but it didn't strike me as a wonderful act of hospitality.
I was reminded of
The Castle of Adventure at one point when a character says, "We're going to need a rope" and (I think) Ben miraculously has one around his waste.
I liked having the war specifically mentioned and kind of wish that EB had done this with more books. Having the two men coming back from the army is interesting, but understandably is another moment that feels a little rush due to EB having to cram in so much plot.
The ending I have to say that I find unbelievable. The police giving children fireworks and the like in other books I can understand, but giving them a motor boat? That seems slightly OTT but maybe the police were that generous (and that reckless) during that era.
Anyway, I hope that didn't come across as negative because those are small faults in the great scheme of things.