Sorry to read what you're going through.
Personally I would contact him, but it would be to lay out calmly and simply the facts of how things are going to be, such as:
-I thought we could use the money we were going to spend on the wedding venue for a deposit on my/your new flat, I think we're still allowed a 50% (or whatever) refund at this stage
-you need to speak to work about changing your working away for three weeks, because I'm assuming once we're split you'll have the kids every other weekend, so you'll need to be home every other Friday to pick them up and then return them on the Sunday.
-Also, do you think you'd be able to use your holiday entitlement to take a half day every wed, so you can come home and see them for a few hours midweek every week? I doubt you'll want to go 2 weeks at a time without seeing them.
-I've looked at the CSA website and it says maintenance should be X each month, I was thinking we could arrange the money stuff ourselves though rather than go through them
-If you have the kids this weekend, I'll box your stuff up and put it in the garage while you sort out a permanent place to stay, or shall I drop it off at your mum's?
I think he really needs to visualise what it's going to mean to his day-to-day life to move on from you and the kids before he'll really get how good he currently has it.
Obviously this could go two ways, he'll move out and not look back, or he'll beg for forgiveness and to come home. But in my mind if he's going to go it would be better if it was sooner rather than later, because at least you know where you stand, and from his actions it seems that he's halfway out the door already.
Hope you don't think this is too harsh, I think you've coped admirably so far, but he just seems to be shrugging his shoulders at you and carrying on his bachelor lifestyle, and you're worth more than that.