Ground under old storage frame

It looked like a cold frame when we moved onto the allotment three years ago. But it was actually just a storage frame with a wooden floor with space under the wooden constructed floor - kept old hosepipes, pots and such stuff. It was used as a safe haven for rats and other vermin when the adjacent two or three pigeon keepers didn't get their act together. They've had a major clearing up of their sites so thought I would join in and remove the "shelter".

Cleared it out and stacked or threw the stuff yesterday. Broke up the flooring which had been eaten by the woodworm in the past - no longer really even solid. That will get burned for wood ash in the not too distant future along with other wood and cuttings loafing around.

But the soil underneath looks as if it's pure clay, full of stones and could have been under paving slabs in the distant past. So any ideas on how to improve it to make it usable? I left it open to the rain last night for starters to soften it up.

At the moment it's not even any use for putting a compost heap on top as it looks too solid to have any drainage. I've some spare old compost from tubs I could work in.  I suppose I could bag up some of the top "insult to soil!!" and get it taken away to the tip but that could be a right faff. Would rather work with it if I could.

Went down this afternoon after the rain yesterday evening had time to sink in. One half of the area was absolutely sodden (nearest compost heap) and the other side near the privet hedge was absolutely dry.

The wet side turned easily enough for wet clay (you get used to solid, wet clay after a time) and the other half had to be watered to get the spade to go into it.  Surprisingly, to a spade deep it's just cold clay and there were even one or two worms.

Do I want to use the area? Preferably - there's a concrete hard standing down the middle of the plot from the gate that takes up enough space without losing any more.

At the moment I've just thrown in the last two marrow plants and the final courgette plant with the idea of building up a layer of compostable material to dig in once it's rotted down.  

You're probably right Goosegirl in that compostable material, manure or whatever is the way to get started.  Can't rely on the three worms I found though for an immense amount of help - reinforcements needed I suspect.  

Suppose I was jumping the gun after asking for advice but it was a nice sunny afternoon you see. 

How about throwing compostable stuff in during the autumn, then growing courgettes or something next year - by then the worms will probably have worked the ground for you?