Hi Simon,
I find, strangely enough that when I've taken on a job that's not been cleaned in a long while that often very old bird muck will wash off with little effort. (not always)
However, often fairly fresh muck (a week or two) will stick like super glue. I've even wet it, give it a little scrub and let it soak for ten mnutes and it's still solidly stuck. and I want to move on to the next job but I can't till i've got out my other pole and scraper and done it the long way.
Hence the experiment!