Posted: 18/08/2014 at 18:39
If you've got good soil in your plot you don't really need to add much depth - just as you say, formalise the edges and neaten things up. I'd say an additional 4 to 6 inches would be plenty.
If you get well rotted manure it'll break down quickly - you can fork it in a bit when you spread it and then fork it over again in the late winter early springtime, bringing up the soil to the surface, and it'll be fine to sow and plant in next season. You can also incorporate some well rotted garden compost and leaf mould if you've got any