What's your location? Makes a big difference, in some instances. Oops, I see that you are in the UK. You good people have more of the tent making caterpillars than we do! I'm in agreement with Larry that it resembles some of the tent caterpillars that we have in the states. This post was edited by rhizo_1 on Fri, Aug 29, 14 at 15:19

Thank you Sarah, this was the closest I came up with, but I wasn’t sure if it would have to have the orange/ red or if this is something that would show up after it grows a bit. It was almost an inch I would say.

Possibly, an Orange striped oak worm - Anisota senatoria

I've raised a number of EBST in enclosures, plastic jugs and re-purposed fish tanks, as well as watched a bunch out in the wild of my backyard: I've never seen any suspend upside down like the Monarchs do, so news to me! Mine always had a preference for suspending vertically, whether on a twig/branch or even directly to the aquarium side (!), but never the roof/top. I was so fortunate to be in the room when my second EBST started its pupation. Here are some stills from the video -- it took almost 4 minutes from the time the caterpillar skin started 'cracking/peeling back' to the time the skin was finally shed. I'll post a few more as a separate comment, in case Houzz limits me to four images.