It is a cabbage but one that, probably due to stress, is flowering already instead of putting its energy into growing lots of leaves. We've had this problem too and think it is because the weather was hot for a while and then went back to being cold. Where are you in the UK Droid?

oh ok, thanks for replying, i live in a small village in oxfordshire, wallingford.
does this means its no good? or should i leave it an get the seeds for next time year?

You can try removing the flowering heads and see if it starts to heart up, if it doesn't I'd be inclined to yank it out.

you could try cutting off the flowers and then feeding it with a nitrogen based plant food [for foliage] and of course, plenty of watering, good luck and don't give up on your cabbages just yet!