Thanks both for responding. I took it that this wasn't something very obvious due to lack of replies! I checked against more common brassica diseases and it didn't quite tick the boxes.
I didn't add the diseased material to compost. It's still in a pile on a slab next to the brassica bed and I will chuck it out when our local waste disposal site decides to reopen again after refurbishment. I will be rotating away from that part of plot next year. Ironic, as it's the first year I haven't had any clubroot problems with brassicas, which I put down to copious liming. The sprouts are a decent enough size anyway, so no problem if they stop growing.
I have had rubble netting over them since they went in, over blue plastic hoops, but the tops had begun to push against the netting since about end September. Our autumn was about average for rain, and not too mild and it's a lot colder up here now so I hope that helps. Not basking in the tropical south....
They will be eaten by New Year anyway
! Red cabbage next to them seems fine - I still have a couple to lift there.