Start watching the weather forecasts in the last week in May. If northerly or northwesterly winds are forecast, hold back 'til early June, otherwise you should be OK. Harden your plants off for a week or two before this by putting them outside on mild, not-too-windy days and bringing them in at night. Don't overdo the kindness though - they're tougher than you think! You could leave them outside all night if the temperature isn't going to fall below about 10 Celsius. Watch out for dem damn slugs!