Posted: 04/07/2012 at 23:54
My first thought would be Amelanchier, mainly because I'd love one, but would have to wait till my neighbour's Rowan goes belly up. Lovely as the Rowan is, it is a bit big for the size of the garden. I think A. canadensis is a bit smaller than A.lamarkii, but they're both lovely. The RHS website says they'd be fine against a north wall.
The Laburnum vossii does look lovely though and not too big at 7m. Re the Crab apples Malus John Downie is lovely. Acer griseum too could be a good choice. Lots of choices!