Do you mean the last of the broccoli from last season or this years young plants?

In a container or in the ground? With or without lime? What kind and what amount of fertilizer? With or without crop rotation? With or without nosy parkers trying to help?! :)

It may be too acid, and/or too little nitrogen, if the compost is commercial instead of home-made. In acid soils, members of the cabbage family are also subject to a disease called club root, which can turn the leaves yellow.

Just a thanks to all that have taken the time to reply to my questions, it's reassuring to know that there is a quick and easy fix to any problems that may pop up, and it has given me a little confidence to try out other things.
Thanks again.

We haven't really solved your problem as you didn't answer our questions. If club root is the problem you will find the root has gone very lumpy and distorted. If you have this do not plant brassicas in the same place for several years and don't walk on the infected ground as it can be spread on your shoes. Dipping the young plants in lime when you plant them out helps to protect them.

Or you might have cabbage root fly, which i've only just remembered. There will be a cluster of tiny white flies round the bottom of the stem at ground level. This can be prevented another year by fitting small circles of something like vinyl etc on the ground round the stem to prevent the fly getting access. You can buy them if you can't make them.

Stera, Mellywong answered by editing her question--not the best way, since it means that the responders aren't notified.