What ever you do Kev, that patch where they were grown will be unsuitable for onions for a few years, Some Jeyes it, some give it a heavy liming.
I think most important is to dip your spade and if possible soles of your boots etc in Jeyes after working that patch.
Once you find another patch that's free from white rot, use that as your onion bed year after year..Some are going to say "what about rotation?".
If you keep your onion bed well mucked and rake some BFB or Growmore in just prior to planting they will grow fine.
