Hopefully someone who's better at the veggie thing than me will answer this, but it sounds to me like your cabbages have bolted...

they are quite leggy and then they heart up towards the back end of the year. they are a late harvest cabbage. so unless there are flower buds forming they havent bolted.

Thanks very much seaburngirl and bamboo. I only started growing veg last year. I have a lot to learn.