You could use a Bordeaux Mixture to protect the new growth and remove the old infected leaves as they begin to die off. If the site is in your garden, I would consider leaving it a couple of years before planting again and then in a different spot. If again its growing on an allotment the spores can be spread from other infected plants (even onions and leeks).

Thanks. My leeks and onions are unaffected so far, fingers crossed.