Wow! I really am talking to the big guns
You guys really know your stuff...thankyou ever so much.
I shall keep paragraph 2.23 etched into my memory just in case..
Well, I have news too. After my email this morning I had a phone call from the admissions lady over at the county council.
She was very clued up too, very fast, had our application right in front of her and understood perfectly without my needing to explain anything. (weird but good!)
She said that this is what happens: in the next few days, the information about all the applicants will be sent to the different schools. So each school gets a long list of every child who wants to go there, with their criteria and so on. then each school sifts through them and makes a list of them in rank according to their criteria. (I didn't realise the school did this! Better keep onside!)
The school has to send this information back to the CC by the beginning of March. (this is all for our LA btw, might well be different for others).
Once the LA has these lists back from the schools, they go through them and allocate places based on what the schools have given them.
Now, this is where it gets interesting. If something happens, such as a family moves house, or we withdraw our son from the school, between March 1st and 31st, the school will inform the LA and depending on whether the allocation has yet been performed, the LA may be able to change the ranking (thus making us a low priority) or may not, if the allocation has already been 'locked into the system'. So it would be a risk.
but if we leave it until we have our offer, the place will be ours, whether we take our elder child out the very next day or not, because it all depends on where the sibling is when the place is allocated. NOT apparently where they are in September - or even where they are expected to be, possibly, though she did not mention this and if it's part of the AC then maybe it is a grey area as to whether it's expected or not?
Anyway, she said the place could not be withdrawn after being given to us, as people change their circumstances for many reasons including moving house etc. But it's all about where they are NOW.
Therefore it looks like keeping ds1 in the school until the end of March would be a good insurance policy, because if the school resolves the issue and the other child starts leaving him be, maybe gets a statement, maybe even is moved to a different school himself - ds1 will have the best chance of going back to all his friends in a familiar setting because he'll have the reverse-order sibling criteria in place.
It's intersting isn't it? I'm grateful that she was willing to spill the way it works, because I didn't understand what we were dealing with before.
Thankyou for all your research and for being so helpful. I think we now have a plan 
Earwicga, yes, tackling behaviour is great, it just scares me a bit as we don't have a system like that where we are - behaviour seems very much under sontrol, the issue we are having is related to SN which is a bit different as you'll understand.
Will still visit other school tomorrow though 