I read it all... 
You are out for the night most of the week? well she is in all month but for one night being the mother and father in that family, yet you feel you are being nice because you care for the children ONCE a month so she can go out?
"Intimacy has never been an issue ? but not as frequent as I would have liked and she says that I have made her feel obligated at times due to her being stay at home mum ? this is your duty type of thing and because it is a lot less frequent than I would like I can see how my sometimes negative comments about it would seem like that."
Type of DUTY??? FGS!
"I have a few bad habits such as smoking and drinking to excess every now and then (ironically a lot more now during all this ? it doesn?t help the situation)"
So... you get drunk, and are getting even worse, but is her fault because of bringing up the idea of leaving you... Charming... absolutely charming...
"She also says that she still loves me (and always will as we have a beautiful family together) and likes me, she is just not in love with me and her heart says that in order for her to be happy she know she has to be on her own with the children, unfortunately for me I am still completely in love with her and love our family"
So she says she doesn't love you anymore, she is trying to let you go kindly... yet you think that her feelings are irrelevant because they are not in your best interests of those of the children... Does she has any say on her own relationship?
"I am willing to do anything to keep the family together and I know that if she leaves she will have to move into a flat, probably in a not very nice area with not very nice schools or people "
Yes, because you have not offered to go yourself, as good fathers do, and leave the children stay in their nice house and neighbourhood. But again, you are saying that she is the one getting the family into trouble because she wants out of the marriage.
"When someone gets to this point can they be swayed, what else can I do to make her realise this is a huge mistake? All those times that she suggested something and I just put it down to being a bad idea or nagging ? "
No, to be honest, she is telling you left right and centre she doesn't want to be with you, yet you think that is irrelevant...
I don't blame her for wanting to go, to be honest, you seem to have acted incredibly selfish for years on end.
If I were you, I would stop putting pressure on her, I would give her space, have a separation trial if you wish, but move out (you! not her and the children). And then try to work yourself back into her trusting you won't be as selfish as before. But remember, no matter how much you say you love her, if she doesn't love you, you can't force her to change her mind. Either she starts loving you again or she doesn't. You have no control over that.