I'll say a couple of things, then give David a chance.
The first point to emphasize is that this rise in the number of lone-parent families, up to about a quarter, has been a very long-term trend. It's tempting to imagine that it is accelerated under the new Labour government here, which has produced a tax-credit system that in some ways is quite favourable to lone parents. But the truth of the matter is that, as in fact with most things in Britain, the era of revolution, the era of great change, was actually in the late 1980s under the premiership of Mrs. Thatcher, after the recessions of the early years. It's almost that period of great growth in the late 1980s that seems to have been when so many things changed, and I think there was a substantial rise in lone parenthood then.
Should the systems be adjusted to provide greater support for two-parent families? I think it's a very reasonable question. A point that is worth making is that most of the in-work poverty for children is among two-parent families; most of the out-of-work poverty is among lone-parent families. I think it's partly a question of how far one wants to sort of socially engineer in this way, or whether perhaps we need to look more basically at the way in which we recognize and reward the caring responsibilities. David has talked about this; we are a society in which unpaid caring is poorly recognized, including financially.
Perhaps the answer to the problem that you raise needs a slightly oblique response, rather than a direct one. But it's certainly a legitimate question.