I finally got into the "garden" (patch of bare, rock-hard clay in the yard) today after having a hectic couple of weeks at work (I got a promotion, too!) and found that the mattock gets through the hard crust and breaks it into big enough chunks to remove. This will leave about 10 centimetres of space that I'll have to fill with organic matter. The soil underneath seems to have a slight clay content to it, but a bit of organic matter should have it sorted out.
I might start with a 2 x 5m patch as my main vegetable garden. Let's see if I can manage that :-)