I have finally found out what's been causing all the bad weather! In Beth Chatto's Garden Notebook she explains that "we rarely escape shrivelling cold weather when the blackthorn is in flower". This phenomenon actually has a name, the "Blackthorn Winter".
If you went anywhere near the countryside over Easter you'll have seen both vicious hail, and blackthorn in full bloom, which does tend to confirm Chatto's folk wisdom.
Though just to confuse us city types, the flowers are actually a creamy white: it's called blackthorn because in the autumn it produces rich black sloes. And if your interest is at all piqued by sloes, check out the site devoted to them - which provides the excellent tip to look for the blossom now to remember where the bushes are come sloe-picking time - the site goes under the unforgettable title of Sloebiz.
(Picture from geograph.org.uk by Kokai)