I read that same statistic someone else posted about 90% of flocks having MG. It seems high to me but then I think of wild birds and just about every yard will have a wild bird visit sometime or another.
I'm sorry you got the bad news Pip. It is one of the reasons I dread the testing part of owning chickens. I have gotten the free egg testing done twice now. Each time I dread opening the results. Come June, I'll get the eggs tested again.
I have my heart set on doing it this year. Just have to remember to send in the paper!
Doesn't look fertile. These are not my pics but here is a good reference: