I didn't realise at the time. I took photos of the other side again at first and was admiring its golden sheen before taking just a few shots of this side for a change. I just thought the dark but was more patterning until I zoomed in when I arrived home.
I'm suspecting it's actually been parasitised but love to know what you think. I'm sure it's only been cocooned for just over a week based on the fact that I found the caterpillar coat on a leaf below. It also looks more like it's been eaten out rather than snapped out.
Hopefully something gets a good life out of it. I might try and collect it next time I go round there.
(Shoulder update: unbelievably heaps better! Pleased (apart from the dishwasher bit.))