Masegwyn,
What sort of onion sets or transplants are they / most can be planted now ( though not all ) ?
I have a shed load of info about early crops and what type etc.to use , let me know what type & I'll see what my oracles say .
I'm at SA18 3BZ , 117 mtrs above sea level the temps have been aroind 48 F average (9 C ) for seven days with only a very ligth frost two evenings ago .
My sets were put in last week after the frost thawed right through the 8 inches of frost in my veg beds so were my final garlics .
Plant your onions in a prepared bed that has been well manured with old composted animal manure , watch out for the earth and birds lifting out the sets ( bloody crows are doing it around here ) , simply push them back down an inch deep and lightly cover them up .
Obviously don't plant in frozen ground & at present there is no need to water them unless your ground is bone dry.