Oh god that last post from Nigel was nauseating and I note that he hasn't answered any of the questions. However, you don't need to answer them here Nigel, but you do need to answer them honestly to yourself if you genuinely wnat this probelm to be solved, as opposed to giving yourslef permission to have an affair.
"Why he no longer tells her how great she looks, why he no longer buys her gifts, arranges nights away (without kids) books romantic meals, helps as much round the house."
What do you mean, helps as much? You said you both work. You shouldn't be helping, you should be doing your fair share. Taht means the mental work of the organising of the household, not just "helping", as if it's her responsibility.
You do know don't you, that one of the reasons so many men have your problem (wife who won't put out, wah) is that many men have this attitude that women are there to serve them. And even though women do it, because we tend to fall into 1950s domestic work patterns during maternity leave, they are left being a seething, simmering mass of resentment about it. We didn't sign up to be skivvies and have the bulk of childcare and domestic labour fall to us, meaning that we can't pursue our careers to the same extent our husbands do, but somehow that's how we've ended up, even though most couples formally subscribe to the idea of equality, at least before babies come along. And that simmering resentment comes out in not wanting sex. It's not conscious - ti's just very difficult to feel desire for someone who is exploiting you.
And if you aren't doing your fair share of housework, if you are only "helping" around the house, when you are both working full time outside it, then you are exploiting her. And that is probably the biggest passion killer there is and it's one that is never talked about because women are told that they are being petty to worry about the balance of work in the household. But research shows that the more housework men do, the more sex they get. And I suspect that that's because if a man is pulling his weight in the home, thus enabling a woman to pursue her career/ interests outside the home as fully as he does, then the woman he lives with knows he loves and respects her, so she feels more desire for him.
You don't say if your wife works full or part time, but there is a crucial measure of fairness as to how much housework people should do that I think Solidgoldbrass came up with and is as useful as any I've come across: If one of the partners in a relationship is getting signicantly more leisure time than the other, then the housework is not being split fairly. Adn you have no idea hwo much subconscious simmering resentment that can cause, mumsnet threads are full of it.
Ignore if not relevant, but I am genuinely trying to help here. Lots of men simply have no idea about this and I suspect that it's probably the biggest cause of lack of sex that there is.