When is dew due?
Spring and autumn—but especially spring—are when dews are commonest, because this is when the relative temperature difference between the (cooler) ground surface and the (warmer) air is greatest. However, dews can appear any day of the year.
Its heaviness also depends on the humidity of the air mass influencing the UK at the time (sea air carries much more moisture than dry Continental air) and if there is any breeze. A very light surface wind can make dew heavier, by causing turbulence that brings down fresh, moist air to replace air from which water vapour has already condensed.