Spring Cabbage Question

I'd wait a bit until the weather warms up and then a bit of sulphate of ammonia can be good.

Wait until it warms up a bit. If you put any of the soluble chemical fertilisers like ammonium nitrate or ammoniun sulphate on now it will be washed out of the soil before the plants can use it. As soon as you see the first signs of growth is the right time. Not too much - about a half teaspoon around the base of each plant. Do not let it go on the leaves as it will burn them, if it gets into the heart it can kill. Choose a day when the leaves are dry. Organic fertilisers like chicken manure pellets are less critical with timing.