Looks like a wasp then. That was my first though but wasn't sure as it's pretty small, maybe a lonely wasp or they like it snug...or campflaged behind the pipe so easy to hide in the hood :)
It is fairly exposed, especially right by a window which my two cats (and spare one) are in and out of all the time and by where a load of wheelie bins are stored.
saw one like it in a loft, just one large wasp hovering about around it. also seen the same looking thing like a chinese lantern in a loft, as big as a football, a beautiful work of art, and luckily it was devoid of the wasp family. so, i would agree that it is probably a wasp of some type, not the common one.
I agree with Ratter, it looks like a wasp's nest. From how it appears to be jammed between the pipe and the wall I would say that a wasp could not have built it there as it wouldn't have had enough leg room so it must have fallen there.