Thanks for your replies, it's much appreciated.
Demented I think this is what I'm worried about. The school hasn't done it before, so I'm worried it'll end up a mess.
Trb17 DS will be a year 2, birthday mid-year and I would say mid-level in class too (although we get next to no feedback at all) so although I can see how the younger ones benefit, I'm not entirely sure how it will benefit the older ones.
Can't thank you, that's really helpful. There will be an info evening for parents soon, so I have written down the question you suggested about mixed seating/grouping as it will be interesting to know what they say. They haven't recruited some of the teachers yet 
Gin they don't really mix at all at the moment - the two class r's do, but years one and two are separate from that and separate from each other with little interaction.
I think my main concerns are how it will affect DS being separated from friends - they've been in the same class for two years and are just about starting to form more solid friendship groups and to settle as a class too. It seems such a shame to disrupt it all.
Also whether, if there are more Y1s whether the lessons etc will be pitched more at their level so the older ones are held back a bit (although I appreciate what can't says about abilities)?
Also how it will affect DS's maturity. He's quite immature for his age and I worry that being in a class with younger ones will not help this!
Also concerned that when they move to Y3 they will have to be mixed again as I assume they can't mix KS1 and KS2?
On the other hand, they'll be in classes of 22 or so, rather than 30 so in theory they should get more attention from teachers etc? Or will that tend to be more focused on the younger ones?!