Any chance of an image? Sure would help us help you.

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

Apatelodes torrefacta I have a little beginner's book about identifying caterpillars that I can't find anywhere! But it shows the most common, most likely to be seen, caterpillars in the eastern U.S., as I recall. And I think it's by Peterson. If I were you, I'd get it, then, when you see a caterpillar that you can't ID, go to that book first. If you can't find your cat, find one in the books that has similar characteristics and look up caterpillars in that family. I guess I'll have to order another one of those helpful little books. The common name of your cat is spotted apatelodes. They come in white and yellow, I've seen both. Sherry

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.