My home is being taken over by ladybirds, but why?

My house is slowly being taken over by harlequin ladybirds. To understand the magnitude of this issue, it's important to first describe my house.

I call it a house but realistically it's a small flat that happens to have stairs - my kitchen has stairs that lead to my bedroom that has stairs that lead to my lounge.

My lounge, on the second floor, shares a window with my bedroom on the first. The wee beasties that are, by now I'm sure, deciding on how best to eat me and my dog, have taken up residence in this window. There are no doors I can close until this whole thing blows over.

My home is in an old building, the window is original and doesn't quite shut all the way. Because of this, all sweeping of the ladybirds is in vain. As soon as I have swept them out of the window, they simply turn around and come back.

I have taken to sweeping about a dozen of them into a dustpan and walking down two flights of stairs to empty them outside the other side of the house. By the time I climb the stairs, another dozen have appeared.

If they stayed in the window, I could probably cope. But they don't. I have found them in my pockets, in my bedsheets, down my sofa - one even drowned itself in my toilet.