Hi Michael,
I have not grown these particular onions but you could sow them in boxes in an greenhouse in January. They will take a few weeks to come up. Then prick them out when about 2-3 ins tall into boxes or pots and take the young plants to the allotment and plant out in April. This should give you a headstart. Don't forget to harden them off first.

I start Kelsae off really early end of December! but that is to get them as large as possible, take what it says on the packet as a rough guide

thanks i think i,ll start them the end of jan