Thanks for that research, Mystery! This afternoon DS told me that his maths teacher had told him that she 'wasn't worried' about him, so, presumably, he's doing something a bit better in the classroom. He does have a tendency to make very silly mistakes and, conversely, to over-think things. When I looked at last w/e's maths homework, he'd got two answers correct but had clearly looked at them again, decided they were not right, had crossed out the correct answers and came up with something completely different and wrong.
I think that all comes down to a lack of confidence in his abilities. He doubts himself. I'm glad I've gone back to the middle of S&S mental maths 2. They are generally pretty easy for him, he gets them done quickly and gets a high score and I can see this makes him happy and boosts him a little. I know we've still got just over a year, but I'm beginning to worry we'll run out of time