In my area during my allotment years I used to plant my onion sets in mid March. Early planting and changeable weather could make them run to seed. Good quality onion sets must be small and firm. I hope this helps.

Thank you so much for your reply. I am a real novice and wasn't sure if there was a difference between the sets you buy in Autumn and those ordered in Spring.

If your onion sets are Japanese onions, then they are unlikely to do well in the spring, as they are designed to bulb up early and be harvested in June or July. Usually they produce a crop which has poor keeping qualities.
However, all members of the onion tribe which can survive a harsh winter are better planted in the autumn, especially garlic and shallots. This is because they create an extensive root system in the winter and can then grow away and make large bulbs in the lengthening days of spring and summer.
Try planting your sets as soon as the weather warms up (it's too late now) and also sow some seed as often seed grown onions are better than sets. Ordinary onions also keep far longer if you want some you can store for a long time.

Thanks once again for the help.