The end of my tether looms!

If your jurisdiction is anything like mine, to all legal intents and purposes you will be regarded as married and will thus have all the legal niceties to deal with in any event.
Such as asset splits / spousal support / custody - visitation matters and the like.
It'd be a real smart move to consult a lawyer so you are fully conversant with your rights and your obligations.

I did that too, that was before I had found iliasm on EP. You can't undo it. 

Might be best to scrub "the talk" at this point Sister E.
Unless "the talk" is conducted from a position of certainty and confidence (in your exit plan) then you are likely to do more harm (by shredding your cred) than good in pursuing it.
Give it a miss for the moment would be my suggestion.
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Whatever energy levels you do have at the moment might be best directed at developing your exit strategy. You may be able to do that in manageable little bites at a pace that suits where you are right now.
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I wonder if a personal counsellor might be of value to you to assist in negotiating the maze.
These situations are quite a trial, very very hard to do all on ones' own. A support network can be very valuable.
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Feeling for you at this trying time.

In principle I support what Bazz and JMX are saying: don't shred your credibility. If you are still interested in negotiating, in coming to some form of change and compromise, don't make threats that you won't follow through on - you just end up looking hysterical and not-to-be-taken-seriously. If you say it, you'd better mean it.

There is a different scenario however: Once you know you're done, once you know you're going to walk out never mind what, it doesn't actually matter jack shit what your cred is with the dysfunctional/abusive/detached spouse. You just have to make sure that you have your ducks in a row in respect of not getting royally screwed over once you actually leave. (and in a few cases you have to also make sure you don't get your head beaten in - I remember one or two of those from the earlier EP days).
Personally, I'm actually in favour of giving fair warning instead of having some bailiff serve papers unexpectedly - but then, I've never been with someone who was so fucked in the head that I had to have fears of getting screwed. My one divorce was amicable, peaceful and mutual. My previous separation was painful, amicable and instigated by me when I came to the conclusion that this woman, the love of my life, was no more reacting to me than a wooden Indian and I didn't want to go on living.

I've never had a belligerent or adversarial separation.

And, eternaloptimism, I wish you the best of luck.

This is a textbook approach in how not to approach a dissolution of the relationship. I'm not being critical, rather this is stating what should be a mindset as you contemplate ending things.

Anytime one contemplates a major decision it should be in a format of proactive steps toward the desired end. It should not be approached as "winging it" after events start to play out. That's why it's so important to see an attorney, stash away as much cash as possible, really shore up your support network, rehearse things in your head, put as many assets and people in your bank as you can, because you may need them. Consider and reconsider making things work as a single parent. Logistics can get pretty complicated when it's just you taking care of and doing all the adult stuff. And lastly have a really honest talk with yourself about where you are headed when it comes time to pull the trigger on your exit.

Perhaps fortune was on your side last night. You have been given another slot of time in which to address the legal aspects and implications. Some leeway to seek advice and research the things that need to be in place before you make this leap. It's really a chance to formulate your best strategy before going forward. Make the wisest use of it. Good luck.

Thank you all. This is all good advice.
I actually feel crushed inside at the moment and my head is well and truly rammed right up there in my sphincter!
I'm less concerned about all the legal stuff. Really.
He knows better than to try to screw me... He's made sure we have never had anything for him to screw back out of me!
I will walk away from this with the clothes on my back and the kids and fuck anything else.
I don't need him, I'm better off without him in every way imaginable.

He can't take me any lower than he has already.

I just need to get the words out to him that his chances have run out. I just need to start the sentence.
Once I start I fear I won't be able to fucking stop.

He needs to leave. He can't afford the rented house we live in, I can. It's that simple.

I very much doubt he would try for custody of the kids or anything like that. He doesn't have the get up and go.

He can do what he wants anyway. Once we are separate he is out of my life. I don't care anymore. I will be fair to him wrt seeing the kids. Anything else and he can fuck off.

I am feeling the anger now.

eternaloptimism, you got this!  Keep your chin up and it will happen.  You've gotten great advice here about seeing a lawyer and having all the pieces in place before you act.  Remember that you are the one with the power and control now.  And from I what I read of you, you are an amazing, intelligent, independent woman who can walk tall on her own two feet.  And you will do just that when you are ready.  

For now, don't be defeated.  Just keep putting one foot in front of the other.  What small thing can you do today to inch closer?  I'm a little defeated myself, so I'm right there with you, and my small step for today is to call a second lawyer.  I have seen one already, but I need to get another opinion.

Be strong!  Walk tall!  It may not always look like it, but women rule.  Don't ever forget that.