The Japanese ones are in the GCs when the spring bulbs come in, usually October. Easy to grow. Plant 2" deep, as the birds tend to pull them out of the ground. If they make good roots before the top growth, this anchors them.
I find them very good, as bulb up in May when the
previous year's maincrop are used up.

I bought a packet of the Japanese ones last year and they were ready at the beginning of this month. Very big.

Thanks all for taking the trouble to answer. I have ordered today some of the Japanese over winter onion seeds despite being told that growing onions from seed wasn't worth all the hassle, but hey ho I like a challenge.

Thanks again, John

Try the Onion Sets as well John. Be on the safe side.