Hailstones form in strong thunder clouds, especially ones with intense updrafts (ooer missus), high liquid water content, and where a good portion of the cloud layer is below freezing.
Hail is most common in mid-latitudes during early summer where surface temperatures are warm enough to promote the instability associated with strong thunderstorms, but the upper atmosphere is still cool enough to support ice � at least until the hail gets too heavy and falls from the sky. The fact that, of all the places in all the world they are aiming themselves at Swaffham, must be telling you something.