Update!
I brought her indoors for around 4 days, 2 of these with a 2kw fan heater aimed at her, and a 250W bulb the rest of the time, fed her bread (with flubenvet) and corn, and she slowly rallied.
I decided to re-introduce her to the flock, which was predictably tricky with bullying, so confined her to the house for a couple more days. The bullying has eased off and although she is being shunned a fair bit she is much better, with the tail spotted being up for the first time today.
Thanks for all help and advice.