Onion white rot
I have looked this up now on the RHS website, and am convinced that I have it at my plot....
There is aphoto on their site which is identical to the problem I found on mine.
This is what they say about it......
Onion white rot (Sclerotium cepivorum)
All members of the onion family including spring onions, leeks, chives, ornamental alliums.
Infected plants often die before maturity. Above ground onion leaves wilt progressively before collapsing, although this symptom may not be so obvious in other alliums. Underground bulbs are rotten at the base and covered with a fluffy white mould.
Onion white rot is a most serious disease of the onion family because it is nearly impossible to eradicate the dormant stage from the soil. It is caused by a fungus which spreads from plant to plant by contact between roots and bulbs, and is capable of surviving in the soil for up to 15 years, during which time it can infect any onion or onion relative planted in its presence.
Practise a three or four year crop rotation. If the infection is diagnosed do not grow susceptible plants in that area of the garden again.
Removing infected plants and increasing the space between host plants will help to reduce the severity of the disease. Destroy infected plants by burning or putting them in sealed bags in the dustbin. Do not compost infected material.
None is available. If necessary, contractors can be employed to sterilise the ground.
It seems I will have to grow my onions in the home kitchen garden in future....!