I just planted out my onion seedlings the other day (day off!! allotment on a weekday, nothing better :) :))
However, this is my first year growing onions from seed, so I can't say I'm offering advice. Just saying, we're in the same boat! There's always spring for sowing, if these don't work. :)
I'm heartened that saddad's only just planted his out. I always think of saddad as someone who does things 'properly.' (saddad if that's not true, don't tell me!) ;D
I'm going to take the advice about covering them, thanks goodlife and denzle. It's v mild atm, but by Dec or Jan will start freezing.