Soil PH !!

Clubroot is a fungal disease, see RHS advice here.

Liming will help reduce the impact of the disease, but bear in mind that it takes time to achieve plot-wide reduction of acidity, and other plants will not appreciate you adding lots of lime.

The best advice is to a) grow clubroot resistant varieties [doesn't prevent it but makes its impact less], b) start your plants off in modules in clean compost until they have a really decent size rootball, and c) dust the planting hole with lime.

Chemical soil pH testers are generally said to be more reliable than the metal probe types.

With my brassicas, I always start them off in modules/trays with clean compost, and then when I go to plant them I always put lime in the hole, and when they're in, I also put some around the the plant itself on top of the soil.

I have a PH tester which I use to read the ground.  I've used the chemical type as well, but when you need to test a whole plot, it can bet a bit expensive - at £2.00 a shot.

All it gives is a ball-park figure, which the chemical test does as well.

Perlka may be the answer, rather expensive though.

Further to Yorkies excellent advice, Digger Dave has suffered with club root for years (well his plot I mean  )

He has successfully grown Clapton Cauliflowers, and is over the moon. I grew them too on a none clubroot plot, and they are great.

I took the advice of posters on here and I have already ordered a packet of Clapton Cauliflowers and also a packet of cabbage called Kilton  that are club root restaurant.
Thanks to all.