heavy clay

Hi Janmac and welcome to GoY. Dig in lots of humus rich compost, sharp sand or fine grit even shredded paper (the stuff you've put through your security shredder). Grass clippings are god to so long as you have not been feeding or treating the grass.

Lime in acid soil, and Epsom salts, can also help, but even better in conjunction with the organic matter. We use gypsum, too, but alkaline clay soil is undoubtedly rare in the UK! : )