when to harvest cabbages ?

At this time of year the ball head ones will start to get a bit mushy round the edges (a touch of frost and dampness) and slugs & snail will have a field day.  They'll keep quite a surpringly long time in the fridge - take off the outer leaves and any other damaged ones too.  If you have somewhere cool to store eg a garage or shed, they are supposed to be alright but to be honest, I've not been too successful (mine in the past have gone a bit "musty" - possibly due to being a bit damp when they went into storage)