you can plant potatoes,then you usualy follow with greens ,but I would give the cabbage a miss this year and get lodes of well roted manure dug in insted,do not what ever you do add sand or any thing like that it will make it worse ,I must write a blog on clay,but I will breafly explain, clay holds a lot more nuterants than soil, but to relise them you need to do the oposit to what you would expect, clay is made up of verry fine grains were as soil is like granuals.an easy way to think about it is ,clay is like flower and soil is like suggar, to make clay into granuals it needs binding togather ,that is what the humes dose, manure ,leaf mole, old mushroome compus, ect'and if it is posable for you double dig it to provide good drainage. by the way sweet peas and roses love it as it is.

To add to Cliffo's advice - if you can get it dug over before the next lot of frost then do - frost is your friend in this situation, as it breaks down the clods of clay. Then dig in loads of compost, well-rotted manure etc as suggested.

ever thaught of going with the clay as a posative and build a natural pond and a nice bog garden .