I could never keep onions for that long. They always turn black & mushy and attract lots of tiny flies. Have you thought of growing them in the house for the green onion tops? I'd rather use fresh new sets in the spring for planting.

The secret to keeping onion sets is to keep them warm and dry: around 26º C, and 20% to 30% humidity. I guess that that may be difficult in the UK.

If they are the over-wintering variety then I would plant out the required amount with the usual spacing. As regards the rest, why not plant them very close together and let them develop until the early spring and use them whole as large spring onions for salads.

Thanks for your suggestions. I think I'll do what Jimmytheone suggests and pack a number of them into a deep trough. Some onions tend to come out very small anyway but are OK.