Goldie81 there is lots of discussion of the standardisation process on the elevenplusexams forum, I don't know if you've seen it too? But the point seems to be that it's not a coincidence that the cut off scores are reasonably consistent year on year (albeit they are slowly going up it seems) because the point of the exercise is to identify a set proportion of children who qualify. Therefore it shouldn't matter if the exam is easier or harder in a particular year, and the scores required shouldn't vary wildly year on year.
I think the highest score required I've seen is 250 (2010 I think?) - but even then it dropped after the initial allocation round in March. The last few years it's been consistent at 246-7.
Just hoping that all ds's hard work will have paid off, he totally loved the school when we looked round (during the school day, which was lovely) and now totally has his heart set on it. We have put our form in and told him there's nothing more we can do for now, just wait it out.