Onion sets ?? rotting

Hi has anyone got advice on my autumn planted onion sets.  They seem to have come to a stand still and when I pulled one up to have a look the skin was dark brown, soggy and just slid off the onion. I also noticed a little white worm like creature, as thin as a thread and half a centimetre long. I planted the sets in autumn last year under fleece, then moved them under a plastic greenhouse cloche over the winter. I'm thinking now they've been too warm and humid in there.  Are they a total loss or can I do anything to save them???  I want to go ahead and pull the lot out and plant fresh sets but should I use a different plot?  Any helpful suggestions more the welcome, thanks Christine.