Not actually a nettle, or even particularly closely related I believe. But the plants do look similar to nettles until they flower. “Dead” in that they don’t sting.
I’m not sure if this was a harlequin, or the melanistic form of the 2-spot ladybird (which has the red and black colours reversed). It was tiny, so perhaps a 2-spot.
A miniature marvel of Victorian engineering: windows in the roof of West Park Conservatory, the rods to open them from ground level, and counterbalances to make the opening and closing easier.