Did you start them from onion sets or seeds? Sets are more prone to bolting.Once that happens, the quality of the onion goes down hill. Onions are biennial plants.

While it is impossible tocontrol the weather, sowing at the correct time for the variety in question is the most important factor to limit premature bolting.

Over-fertilizing can also contribute to bolting - if onions are too vigorous, too early in their development, bolting can result.

Thanks - they were grown from sets and planted in beds which had been heavily manured for potatoes earlier last year so perhaps it was all a bit too rich.