It is a truth statistically confirmed that a man in possession of a good fortune ought to marry at the age of 29. It's as simple as shooting battleships or queueing in a supermarket.

The standard mathematical problem concerns a submarine commander who has only one torpedo and wishes to fire it at the largest battleship in the enemy flotilla. They are sailing past him one by one, and once gone may never be caught up.

The optimal strategy, it turns out, is to let about a third of the ships go past, then fire at the first ship that is bigger than anything you've yet seen. It's the same in a supermarket: if you want to join the shortest queue you should walk past a third of them, then join the first one that is shorter than anything seen before.

To select a spouse, therefore, one should let around a third of the potential spouses sail past then go for the first one better than anything seen before. Men, whether in possession of a good fortune or not, tend to marry between the ages of 20 and 48. They should therefore let around nine of those 28 years sail past before shooting their torpedo. And 29 is indeed the median age for British males to contract a first marriage (and 26 for females).

What should a submarine commander do, however, if after nearly 13 years of waiting, he suspects that the largest enemy ship sailed past in the first nine years? As time runs out, and smaller vessels sail into his panorama, should he fire his torpedo at the gleaming and previously unmanned Battleship Diana, or dart back to the shorter Camilla queue?

If queue-hopping enters the equation, however, he should not have waited so long before firing his first torpedo.