Just back from checking her out, she seems well considering.
Thanks for the input everyone. I splinted it very shortly after the break (clean seeming, no cut) so the leg is immobilised. We don't have the means to just kill her there and then so have decided to call the vet tomorrow and take it from there. I wouldn't say she was in mega pain, it;s hard to tell, but for example when I;ve trimmed footrot over zealously they can growl and buck, but she has been very placid throughout bandaging and splinting.
I know my job isn't going to be textbook but it does the right thing in keeping it from moving so I'm hoping for the best. At the very least hoping that she's not too uncomfortable poor thing. I just suddenly feel very underprepared- which is a bit scary.