I'd be wary of planting out onion sets to over winter in north Yorkshire. We buy ours later into the year from an organic supplier and do not plant out until early spring. If you want to try thoroughly dig the soil and add good well rotted manure or compost and the plant your individual sets with a good foot + between the rows and 6 inches apart. Be prepared to protect if you have a bad winter. If we tried they would just rot in the ground.

They do make quick green onions, though, if you still have a few weeks with no more than mild frosts. Used that way, you can plant them much closer.

Hum North Yorkshire... could get frosts within the month unfortunately.

Drat, Dab would need a greenhouse or conservatory, then.