There is one thing which will act as a sort of quick `pick me up` on sticky, neglected clay, and that is Calcium Sulphate or gypsum, which can be applied now, and is sold in garden shops under several names such as Clay Breaker. If you have not already dug your plot, then apply the calcium sulphate to the surface and dig it in. If you have already done the digging, apply less to the surface and work it into the top few inches. Either way will result in increased precipitation, or flocculation, of the clay particles and a better crumb structure developing through the summer.
Having said which, although your clay (if it is clay, and not marl) should be on the acid side, it is a good idea to do a few pH tests first since calcium sulphate does have a marked alkaline reaction on the soil. And DON`T use it where you plan to plant potatoes this year
