Chuck it all in, but don't expect rapid results. Keep adding manure every year then after 3 years you will be getting there. Make up some raised beds over the worse bit for as many of the usual salad crops and plant spuds and brassicas in the other. It might seem like a long time but it will come. My allotment site was newly opened 4 years ago, one of my plots which I took over 18 months ago was heavily manured and is now a nice spit deep in good friable topsoil. The other was never used properly so I am slowly breaking it in from brickworks clay to the same as the good one. Still usable but hard digging.
One trouble with clay is it does not drain so well, but if you trench the edges and half fill them with all the bigger stones you dig out, then gravelly soil over the top it will provide what we call a French drain. Perfect for curing wet claggy conditions. No need to dig a trench all in one go, start at the lowest point and gradually work up the plot over a few years, it gets rid of all sorts of rubbish at the same time.
