Onions from seed?

For white onion sets, I've bought sturon and 'stur bc 20' from Bridgend Nurseries in Scotland somewhere as they sell a lot of seed potatoes also. I've had them for a few years but didn't keep separate to see which kept the best. I was going to last year but they arrived in paper bags that burst and mixed in the box so I couldn't. What I do know it I still had some that were reasonably firm on 1st July this year when I lifted my autumn planted sets. They both have a reasonably strong flavour.