I'm not sure it is 'just' lust. There is more in there in terms of specific characteristics she has, which may be linked to some of the questions you ask there.

The biggest thing missing for me is 'intimacy' - since we have had children my wife does not get motivated by physical closeness in the way she used to, and I dont just mean sex, I mean all aspects of physical intimacy that you share with your partner - touch, kissing, affection. We have sex, but it doesnt set her on fire like it used to, and she just isnt as 'feminine' as she was.

That sounds harsh, but its not a criticism of her. She is a great mum and when we discuss this she says herself that she is more focussed on being a mum than a partner. She is a woman who is 'on a mission' pretty much 99% of the time. And that means she doesnt have the mindset to chillout, relax, get in touch with herself, and then get in touch with me.

The 'other woman' here does exude femininity, and has certain ways about her which perhaps I have latched onto as a fantasy of that intimacy I crave..........

The more I think about this, the more worried I am that this actually isnt a crush its indicating more important issues with our marriage which I don't think are easily solved.................