OP - I'm the parent of the twins mentioned upthread!
As mentioned, one twin got in to grammar, the other just missed out. (We'd moved closer to the grammar for one twin and an outstanding comp for the one twin. The comp place was also lost and won back again two months before school started, but that's a long story).
The twin started comp ladt Sept. After a week he was offered a place at a super selective (a different school to his twin).
He wanted to stay at the comp, as he was happy and had made friends - we respected his decision.
Very soon after, things changed. There was low level disruption in lessons, he found lessons unstimulating. There were a few boys being mean. His twin was having a great time at gs, meanwhile.
We tried to get the gs place back that we turned down. It had gone. I kicked myself every day.
My son started to make some nice friends at comp and this was reassuring, but he still wasn't happy in lessons. He's a sweet boy and wouldn't complain, but when I asked him how it wss going, he said the top set wasn't like the top set at his primary. One day I saw a piece of homework and was shocked at the top set level.
Meanwhile his twin at gs was flying. It was hard.
In Feb this year he was offered the gs place again. I asked my son and there was no hesitation - he wanted to go!
Six months later...
He's happy and doing well! We are so pleased we did it and so is he.
It's not all plain sailing though.
The logistics are harder - his comp was up the road and the gs is 7 miles away.
We don't have the same locality of friends as he had at comp so their socialising is mainly XBox (we make sure he sees his comp friends and primary still, and he has his brother and Scouts etc.).
All things considered though he is happier at his new school because he feels stimulated and he settled in well and has nice friends there.
I do sometimes wish he could walk to and from comp with his friends, who'd live locally. That's the only regret.
We always made it clear that if he was ever unhappy he had options.
There is no perfect solution. We are lucky it has worked out, but I never underestimate the value of being "local" with a comp. I'd advise anyone to judge each case individually.
So OP, if I had my time again, we'd have probably moved last Sept to gs instead of Feb, so good on you.
I can't tell you how much regret we had after turning the place down!
I was so happy to get a second chance and would make the same choice again.
I hope your son settles in well. It's so early that he really won't notice.
Don't worry about the scores - my son's was lower than his usual performance but he's doing just fine now academically.
It's a tough decision and all we can do is make a decision that is right at that particular time.
Good luck and I wish you well (sorry for essay).