So I did this cool little tutorial over at Amy's Creative Side and I've discovered it's not my sewing machine (the seams really are exactly 1/4") - its my pressing that's the problem. And if I press the seams OPEN instead of to the side, things seem to work much better - so that's what I'll be doing from now on me thinks.
Anyhoo, I'd already made these two blocks before doing my little experiment so they don't measure 9 1/2" as they're supposed to - so I think I'll redo them and make these into a cushion (cos I really love them despite their not-quite-perfectness)
Simple Fives (Yosegi gokuzushi)
Simple pairs (yosegi nikuzushi)
I'll hopefully have some accurate blocks done tonight!