someone else wrote:
Until recently I had an area at the back of my garden which used to
be
a patio but which I would prefer to be vegetables. So I have duly
removed the concrete and hardcore and am now left with a compacted
clay subsoil which I want to get back to life.

This is a serious clay so it it very compacted and very effective
at
getting waterlogged but from what I have read clay although hard to
work should be fertile. So far I have double dug the ground and
added
sharp sand (about 50 - 100 Kg / sq.m) and composted manure (about
200
l / sq.m). What else should I do to get this to something that I
can
grow on

Wow! that's quite a lot of sharp sand!

As long as it gets sunlight, and isn't a "sink" for surface water, it
sounds to me as though it'll now go like a bomb.

The composted muck probably won't have much actual plant-food in it,
though, so you could prick in some balanced fertiliser, organic or
not according to taste.

You may also need lime, but I'd wait to see how it goes first.

If, after all your impressively devoted attention, it's still a bit
cloddy, you may need to make strips of finer structure for sowing
seeds direct. Shouldn't be a problem for transplants, though. It's a
great thrill doing a new patch, isn't it?

Mike.