I would check the crop tonight when she goes to roost, then first thing in the morning before she eats/drinks. If that has not emptied and is not flat in the morning, then I would consider her having some crop issues.Thank you so much for posting such a detailed post and yes I am in the UK. That's great to hear about the mobile testing service, I will look it up and get a sample set off asap. That's a very reasonable cost for doing it too.
That's very insteresting about the seed/mix, they don't get a lot tbh. Probably half a cup a day between the five of them and that tends to be at the end of the day but having read what you have posted I will definitely ease off the mix and look at sourcing a higher protein layer pellet instead. I can also supplement them with meats.
Before I saw your post I actually took some picture of the hen in question whilst I was just out cleaning the coop. Wyorp Rock had mentioned whether the crop emptied overnight. I was looking at her again just now and her crop (I think it's that and not lower down) does seem a lot more prominent compared to the other hens that I have. I have posted some pictures below and wondered what you all thought. It is more swollen/pronounced than the other hens but what would this ultimately indicate?
She has quite a distended crop, it could be considered pendulous if it's not emptying all the way.
Read the following articles and let us know what your finding are upon examination of her in the a.m.
Hopefully you can get this resolved, if she is having a sour/yeasty crop, then that can play a part in having a dirty bum.
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/
http://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments