Sorry to hear this.
I stopped growing autumn planted onions for this reason and now my gardens are mildew free (touchwood!) and I suspect it is mildew causing this, which can then infect spring sown onions. Currently my spring sown (and planted from sets) onions are healthy.
Is anyone else having problems with overwintered onions?
PS Ladbrokes, summer has barely begun, hang in there! better next week even if not hot.