Thank you AnnaB. Your hen looks perfectly settled and comfortable in her saddle.
We tried with two of us today - the hens went berserk

We don't handle them much so they were most indignant. We eventually got the first butterfly on after a couple of (long drawn out) tries, but we came close to divorce with the second hen

. Eventually I gave up with the butterfly and put the plain saddle on instead (on my own by now) with a much more settled hen, after spending ages calming her down. That fitted her fine, although I found it very fiddly. So I've emailed Flytesofancy for their advise on sizing, wondering if the second butterfly was accidentally a larger size. Although they are adjustable, only for so much, and the flaps just wouldn't stay on properly. I think we'll check the tapes in a couple of days to make sure they're not digging in. After a few minutes they both were perfectly happy and wandered off. So one is well protected, but the second still has exposed wing tops. Also it's recommended to spray the butterfly with waterproofing wax, but I was a bit exhausted and forgot today. We'll get her while she's sleeping. The other one is apparently waterproofed already.