The boxes went on the cupboard at the end of November / start of December, so that's two months, maybe two and a half, for it to spring up; I'm guessing they got pressed up against the wall, and then a lack of circulating air over the winter plus condensation on the wall gave good growing conditions.
So, this afternoon, I'll be throwing out the remaining boxes - the contents are fine, there's no food in them, just the outside cardboard is vile - and cleaning the wall. And then, I suppose, start inspecting the rest of the house - where there's one patch there may be a dozen. Bleh. Well, I suppose it's almost time for spring cleaning.
(Any useful advice on dealing with mould? I've usually encountered it on windows and wooden surfaces rather than walls...)