You could plant them again, but if you want some good productive onions, I suggest getting some new stock. Do they sell onion PLANTS where you are? These tend to do the best rather than sets.

I agree with wayne -- onions take up enough space and are enough work, and are cheap enough to buy either seeds, plants, or new sets, that it would be worthwhile to know that you're going to get a good crop from them. OR plant some of what you've got and some new, and compare.

I don't know where you are and exactly WHEN you planted your onion seeds. But one possibility is that you planted "Bunching onios". Fro what I have heard BO are developed NOT to bulb.

Then there are different onions for different latitudes. SHORT DAY, INTERMEDIATE DAY and LONG DAY. .

I have learned that growing onions is not as easy and simple as it appears to be.

I planted seedlings I grew from seed last year half at my garden and half at my dil's.. hers did great. Mine not so much. Thinking less Sun.

The OP is in the UK and planted onion sets not onion seed. They weren't bunching onions. As far as I know onion plants aren't sold in the UK, at least I've never seen them. However, I have seen seedlings.

Personally I have given up trying to grow onions. My soil is heavy and cold and for the quantity I use and the space and time they take up they have not been worth it.

However, if you want to try again I would buy new sets, or maybe try seed and follow the RHS instructions.

Here is a link that might be useful: Growing onions

I have grown onions all three ways:

1) from seeds. They did not get very big (maybe wrong kind)
2) from sets: Those are tiny onions. In my case a lot of them bolted.
3) seedling or plants: Those are grown in greenhouse, then bunched up, the tops cut and sold.. I have done that fairly successfully. Almost no bolting.

Another thing is that as they say "onions are heavy feeders". You should treat them as "leafy" vegetables, because the bulbs are just continuation of the leaves. So they need a lot more Nitrogen than "P" and "K".

The tricky part is to plant the varieti that is right for your climate.

I've been successful with Sets so far. I planted them in mid to late March last year, and pulled them around August or so. I got some pretty big ones, and they were sweet onions. I got about 80 for $3. I'll probably try some Red and Yellow and maybe White also. We'll see how many I buy, I probably could grow 300-500.

The biggest thing with the Onions was to keep them watered enough and weed every now and then. Once I popped in the sets, they were easy to grow though.