The answer, I suspect, goes back to shortly before Christmas when I was chopping down chunks of the trees out back to let more light down into the back garden. I chopped lelandia and laurel, neither a likely food-plant for an overwintering egg or larva, but there were also passion vines strung among the branches. A delicious evergreen food plant strung among cosy evergreen trees, facing the morning sun? What could be more congenial for an overwintering butterfly egg?
Chopped and tucked into my green recycling bag, it hatched in the warmth of my shed, and gorged on chilled but still tasty leaves, away from the frost and snow while for almost a month my green waste lay uncollected, recycling lorries kept at the depot for fear of ice and snow. Like a caterpilllar in a jam jar it grew fat and fast.
The morning they finally came to collect the green waste, though, was brutally cold. Shaken from its cosy bag it chilled and died. By the time I got home in the evening, something had made a meal of it. Mmmm! Caterpillar slush-puppy.
What species, though? I'm guessing one of the commoner whites.