Could Rusty have kidney failure?

What has she been eating recently ?

Sounds as though their diet is a bit rich and varied compared to what they are used to. 

Is it only one girl who has the tummy problem ?

I've noticed one of my hens passes a lot of water when she's had too much cabbage or lettuce.  I cut all veg out for a few days and she eats pellets and grass and is fine again.

I agree with AS...you give them lots of different stuff. I would cut back and give them the ex-batts pellets adlib, obviously oyster shell and grit and lots of fresh clean water . Mine have a little mixed corn at t-time, and as we have no grass left some curly kale or similar a few times a week. Its very tempting to give them lots of treats and goodies, especially when they first 'come out' because you feel so sorry for them and they need to build up their strength. You really wouldnt believe what i used to give them....it makes me shudder to think about it now. believe me, just because they go mad for it, dosent mean they should eat it.I use a few meal worms when trying to tame newbies or during a moult, but thats it. diet affects them dramatically!You really are doing them a huge favour by NOT giving them lots of things. The ex-batts pellets have everything they need in them. They free range so they get greens from plants and grass, and protiens from bugs and worms. Remember, slugs and snails can carry the gape worm parasite so you need to be vigilant with an effective worming program like flubenvet.
My girl has been diagnosed by a vet after lots of test then finally a poop sample was sent away. I havent been offered anything for her, so she just potters around quite happy for now. if she starts to show symptoms that she is suffering we will have her PTS . I would be taking AS advice and looking at their diet first

As far as I am aware it is not poisoness to humans or dogs and can find no mention of it being so with chickens.

Aha! Yes, I'd got into the habit of letting them free range straight away as, when I'm working, I have to shut them in their run before I start work early-mid afternoon. The new girls haven't been closed in at all apart from their first 2-3 days, then I round them up for bed as the light starts to fade.
Right, from tomorrow they all stay in for the first couple of hours - maybe then they'll all get the proper dose of Flubenvet as well!
I've previously been advised to put Herban in their water but they're not keen on drinking it when there's fresh about. Do you suggest I keep adding it to the water or give them fresh? Our tap water is borehole so nothing hinky about that.

I picked up a pot of Avipro Plus from the vets yesterday and have been told to give it to her as per stressed bird dosage, following instructions on the pot. However it doesn't say how much a day I should give so I made up 100ml which she drank about 1/2 of - the other 1/2 she spilled! She drank what she had like it was going out of fashion.

Of course the vets are closed today so what I need to know is how much should I give her a day?  I was pleasantly surprised she drank it out of a small bowl I was holding, surprised because she won't yet eat out of my hand...

I also gave some to Spike who was a bit down yesterday, and has recently been on anti-b's so I thought it would be good for her.