Thanks everyone for your responses!

I always thought DS had obvious ADHD as I constantly have to redirect his attention when he's doing homework and his teachers have reported the same thing, however he will sit and concentrate on things he's interested in. Also, if he hasn't been outside and burnt off energy, he can be bouncing off the walls.

It does seem difficult to get his attention when he is really into something but I could say exactly the same about DD - and DH come to that!