Celtic Cabbage

Morning everyone & happy Easter 
I bought some Celtic winter cabbage plants from a G/C last October & put them in the veg plot
expecting them to be ready to eat by now but they don't seem to be growing any heads. They look
very healthy, about 24" tall with plenty of leaves but as I said no sign of any heads forming.
As my veg plot is only 3mtr square I am soon going to need the space there taking up for this years
plants which I have growing in the greenhouse.
Was I to late putting them in ? in which case why where the G/C selling them.
Any thoughts gratefully accepted.
Keith.