We're back from seeing the vet. When we got her out of the basket, we saw that she'd produced a rather large, solid poop, so that shows some food is getting through (we removed it and then she did another one on the way home).
However, Mark noticed she was very swollen between and behind her legs, so he checked for fluid. There wasn't any, so he diagnosed peritonitis. This adds up because I've noticed the shells of the two eggs she's laid were uneven.
Then he said when chooks are not well, they often eat too much of the wrong thing and impact their crops. So it seems the impacted crop is secondary to peritonitis.
As her food is slowly moving through her, the impaction will be treated by reducing her free ranging time which will reduce the amount of grass while increasing the amount of pellets in her diet.
The peritonitis is clearly not great as it withstood last week's course of linco. So, she's on clavulox shots for a week and if that doesn't work the next line of treatment will be a hysterectomy.
I also asked about probiotics and he said the vetafarm probiotic was a good choice
@LozzyR . I've ordered some and it should be here this week.