Izzy, I always grow stocks over winter.
I don't know how hardy they are elsewhere but for me they provide good colour and lovely scent on fairly large grey bushes into the spring. For some reason mine this year are either white or purple......love the white ones and I think the scent is stronger. They make nice addition to wallflowers or any other planting you may have.
I think there will be a problem if growing on acid soil........suggest adding lime or adding mpc around the roots at plantImg time
If your plants are in the greenhouse now I would hesitate in plantImg out now. It's pretty cold at moment. However, if you have some fleece I would cover for a while to help them acclimatise. Avoid wet soils too I think . Are your plants fairly large or are they still seedlings? Would plant seedlings out in spring