You need a lot to make any difference, don't use builders sand. Broken old bricks are good, as I've said before, clay to clay.

thanks doc

Adding plenty of organic matter in the form of well rotted garden compost or well rotted manure (a bucketful per sq metre) will probably help open up the soil more. It will help improve the drainage and add some nutrients too. Add it to the surface as a mulch and let the worms take it in to the soil gradually or you can lightly fork it in to the surface layer.

I always use sharp grit from the garden centre as well as lots of manure type things as mentioned by Rogerbee, and have found that to be more effective.

thanks all