Why in the greenhouse, Elizabeth? Overwintering onions will grow perfectly well outside in a normal UK winter. Buy some sets now ('Radar' is a good variety) and plant them just like summer onions, and by next May, you will have very usable onions. Leave them longer and they'll be bigger. They certainly fill the gap before the summer onions are ready. Watch out for bird damage over sinter, though, cover them with wire netting!