Is this a mineral deficiency?

[Stephen]

On the pale one of these brassicas?

[snooky]

Or is it one of those ornamental brassics gotten mixed in with your sowings?

[tigerburnie]

Is the soil alkali as brassicas like a bit of lime?

[Geoff]

I think something has attacked the root, either beastie or club root.

[Stephen]

I think these brassicas were in one of the KG giveaways (but can't remember for sure).
The soil is clay over chalk, so likely to be alkaline. I find it odd that in two sowings in two locations (one here in Berkhamsted, the other in Hemel Hempstead), with all the seed from the same packet, there is one plant which which has all the pallor while the others look so healthy.