Has anyone else experienced getting clubroot from propietary compost in their brassica plants?

No and I sincerely hope it is not possible.
What kind of Potting compost are you talking about?

I would have thought that as club root is most prevalent on badly drained acid soil, that it would not be possible to get it from commercial potting compost, Derek.

I agree how can you get club root from potting compost?

This is a quote from the Vital Earth web site. I use their compost as it has been very good so far.
"The more persistent (temperature-tolerant) diseases are “finished off” by prolonged biodegradation during the maturation phase. Even plant diseases such as Club Root – that have tough resting spores – are destroyed. Furthermore, green compost can suppress some plant diseases, e.g. bacterial leaf-spot diseases and Botrytis."

http://thegreenergardener.com/products/faqs.html

Are you sure it was Club root?

I grew some Kale in a container in the greenhouse last autumn. It grew ok but when I pulled it up this spring one plant had clubroot.
It was grown in New Horizon peat free compost only and from seed so I can only conclude the compost was to blame. Unfortunatly I still have several bags of this compost and had used some spent stuff in the garden before discovering this so I'm worried since I grow a lot of brassicas. I even bought more this spring before discovering the clubroot and that seems even less composted judging by the large amount of undegraded white wood in it this year

I would suggest that you get in touch with New Horizon and ask them what has happened.
Did you buy in the plants or grow them from seed?

Update on last post. I sent the kale root to Sinclair and they tested it. They found it was infected by turnip gall weevil larvae. I never heard if it before and I think I opened a gall and found no larva myself but this is a possibility. It's getting into a closed greenhouse and infecting one plant out of about
20 also seems unlikely but again possible. Apparantly it is easy to confuse with clubroot and is common in southwest England. I am in SW Wales but I guess it's possible anywhere in southern UK, although I never saw it before.

Possibly Portzed also had this problem. I would advise dissect each gall to see if any larvae are there before assuming it's clubroot. I can't check now since I sent off the whole root.

In any case the New Horizon Compost seems to have declined in quality lately, with a lot more undegraded woodchips and also things grow less well in it

There is no doubt in my mind that that the increased use of recycled green waste as a replacement for peat is leading to a general deterioration in the quality of composts. I am the president of our allotments association and I regularly receive complaints from experienced gardeners that the quality of plants grown in the latest composts is not what it once was. I have also noticed that the grain size of the composts is getting bigger and that the quality batch-to-batch varies. With the legislation moving towards NO PEAT in composts available for retail, this problem threatens to become worse.

I am also aware of a court case where a professional grower is suing a compost supplier having suffered serious plant failures.

If you are like me then you only send green material that you would NOT put in your own compost bin to the municipal recycling site so we know that it could contain a high proportion of diseased or contaminated material.

Thank you to Jonock for the information on possible turnip gall weevil having an appearance similar to clubroot. I am in the SW near Bristol and doubt, like him, that weevil was the cause. Coincidentally, or not, my compost was also from Sinclair, under the trade name, Arthur Bowers. When I sent them photos and information on my problem they just replied that it was impossible for any diseased material to be in their compost. How is it that in previous years when using the same brand, which used peat, I never experienced problems?

I am still convinced that these composts are variable batch-to-batch and subject to having diseased or contaminated material in them that is detrimental to good growing. I also believe that recycling is to be encouraged but that the processing methods and their control need to be improved. At the moment I fear that it is just money for old rubbish, literally.

If you have a problem give the supplier a chance to redeem themselves but don't be shy about giving them some publicity.

I had a long talk with one of the head gardeners at the Edinburgh Botanics and he explained that the way the council, or those to whom the green waste goes to process even cooked food and bones are properly composted and safe. Bulba and I are still not convinced and a something like a chicken carcass still goes in our wheelie bin. We used to be able to buy composted bark and that was great stuff but a lot of what is available now is, as already said, very variable.