I'm surprised your cabbages looked okay if they had club root normally I would have expected small stunted plants. Sadly once you've got the problem it isn't possible to get rid of it there are no chemicals that can treat and because it affects all crucifers (including weeds) it is easy for the cysts to spread. They can also live in the soil for a good 8 years waiting for a crucifer to appear.

However you can minimise the problem. Don't compost any of your brassica roots put them in the wheely bin or burn. Don't use mustard as a green manure as this too is a crucifer.

Start your brassicas off in seed trays using bought in compost to which a small amount of lime has been added and pot up to 3" and then 5" pots before planting out. This allows the plant to develop a good root system prior to infection.

Club root like wet acidic soil so make sure you use plenty of lime and aim for a pH no lower than 7.5 higher than that if possible.

Check out the brassica varieties that are resistant to club root. Kilaxy is a club root resistant cabbage.

If you can put up a photo of the root so we can confirm whether it is club root or something else.