Hope thats of some help Tracysshed
I plant onion sets mid March to mid April, depending on the weather. I find Sturon keeps until June and Red Baron until April the following year. I start harvesting in July, so I rarely buy onions. But Summer here in France is hotter than in England.
I always start all onion sets off in modules in my unheated greenhouse and plant them out when roots have developed. As well as ensuring a good crop, it stops the birds from pulling them out. Never had a bad crop since I started doing that.