Thanks for your reply. That sounds very reassuring. But it's not much extra work. Are they at prep schools or super bright? I like to think of my DS as very bright, but he isn't conventionally consistent in his work. He makes many slips and mistakes, and his school hasn't drilled him in multiplication etc. He can work it all out accurately, but slowly.
I can't help thinking my boys will both need quite a lot of prep to reach the right standard.
The Kids I know are all state educated, so the ones that made it in without any tutoring did not have any benefit that may be perceived as being gained from a prep school education. It has to be said that most of them would be deemed 'gifted' but then gifted just means in the top 5%, and the Grammars are basically full to the brim with the top 5%. Having said that they are all, I suspect, fairly comfortably in that top 5%. I'm pretty sure that DS1 is also in that area and likely my other 2 are as well - we'll see!
We have switched to prep school for DD & DS2 recently but not at all because we anticipated that would give them edge, and from my personal experience I don't think it does at all.
I also think that going to a Grammar is not the most important thing in life. It was clear that DS1 would be happiest in that environment, and highly likely that he would be quite unhappy at our local comp, but that's not the same for every child. Many would be just as happy being schooled outside the Grammar schools, many would be miserable in the Grammars.