I always thought that adding lime and making sure brassicas were very well firmed in when planted helped the prevention. Crop rotation helps too. Can't help any more im afraid.

Once you have club root in your soil there is no actual 'cure' the RHS has some good advice on how to avoid http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=128

The only `cure' that I know is time. The figure often bandied about is 20 years.

If you can grow the brassicas on another piece of land. Test it for acidity and if necessary lime it.

When transpanting the seedlings dip the roots in lime - sounds unlikely but it does help.Then sprinkle a ring of it round the plants. Avoid walking on the infected soil as you transfer the disease to other parts of the garden.

If you can plant elsewhere do so but still take these precautions, especially the bit about transferring it on your boots. (And if you should grow Siberian wallflowers do the same as it affects them too.)