That said, she'll still happily go for walks, etc. This afternoon, she was very keen to walk (even doing her old trick of opening the back door with her paw) and then once we were in the park, rolled herself totally in shit so she was not just coated, she was armoured in it. I had to wipe her with newspaper, then sponge her with warm water, then douse her with water in the back garden, then wash her in the bath, then dry her and wash myself. She was really happy about her bath, as well (though in truth she probably would have been happier to be allowed to smell faecal longer).
Our vet, who is usually good, doesn't know what's wrong with her, and this afternoon I called another vet surgery who said if I was worried I should bring her in but reading between the lines from their point of view the real worry was that she might become dehydrated. But she is drinking a lot of water.
She's 9 years old, which is fairly old, but she's always been pretty hearty up till now, so I'm feeling positive.
(This is Charlie a few days ago. I have another picture of her happily covered with faeces and leaf pieces, but I thought that was a bit too special).
Update 2:46 AM the next day: Charlie ate her dinner with gusto, and did not vomit. She might be turning a corner. She is sleeping on the couch next to me as I type this and we are watching Rage.