Thanks Foxy - great advice there. And a relief to hear that e-coli would make her seem under the weather - on the contrary, she's actually looking completely fine.
We just had a lovely cuddle outside in the sun - she fell asleep on my lap so I had a good opportunity to give her a thorough examination. Vent looks clean, feathers tidy and glossy, crop nicely full (but not over full), eyes bright (when she wasn't asleep!) and comb red, and body weight feels healthy. I couldn't see anything outwardly wrong with her. She has not laid an egg since Thursday morning, but that's not necessarily of concern to me yet, as she usually only lays 6 days out of 7. Her bottom feels a little rounded, which I'm hoping is in any case suggestive of a waiting egg...
I already use ACV, but tend to only have it in one of the two drinkers, as one of them is galvanised and the other is plastic. She may prefer the galvanised drinker, so I shall have to ensure she takes water from the plastic one too from now on.
I also give occasional (twice a week) Respite (garlic-based supplement) and Ropiadar (oregano-based) in their water, and they seem to really like a splash of Battles Poultry Drink too. I top up their calcium levels with Davinova C if I notice anything strange about their eggshells (especially as our other hen, a bluebelle, had Infectious Bronchitis, so she went through a nasty bout of laying soft shelled eggs, which sometimes burst inside her and made her quite poorly - she's on prescribed calcium supplements from the vet now, but I do like to sometimes top-up with soluble calcium in their water too.)
Anyway, the only thing to note is that our buff is certainly immuno-compromised, having had mycoplasma a couple of months ago, which is why I'm always watching her so intently! Perhaps I just need to chill out a bit!
It seems that I should suspect the cause of the extra caecal poo is all that olive oil then - a change in diet. And the funny thing is, she hasn't done one today yet, not that I can see anyway!
Will post a poop pic if I can catch her in the act, but fingers crossed she was just full of olive oil and it has worked it's way out now...
Thanks so much!