There's still time to plant this years spring planted sets or onion plants can be picked up quite cheaply at garden centres just now.
To make sure of some onions I would cut my losses and just give the Autumn sown ones up as a bad job!
If you dont need the land they are on you could just leave them and after taking the seed heads off use them as quickly as possible!
I must admit, I have never had this problem with autumn sown onion sets but I always have problems with red onions from sets going to seed!