Thank you everyone - not a happy ending to this one, I'm afraid
I was still worried after speaking to the vet, so called my friend at the farm, who kindly came over, swore, told me I'd been poorly advised by both the person who was helping me dose them on Sunday and the vet, and then as she was on her way into town anyway, said if I rang the vet to authorise her putting things on my account, would get me everything I needed, but prepare to probably lose the poor one.
She came back with Extravite, pen and strep and kaolin for the thin lamb plus antibiotics and painkiller for the limping one, who we found had heat in her knee on top of the touch of foot root, and walked me through the injection schedule. Sadly late last night the thin lamb decided she was going to stop drinking, lay down and refused to even try to stand. She surprised me by making it through the night, but with no improvement today, after a chat with the vet this afternoon when I was getting the puppy's second vaccination, we made the decision to send her on her way rather than letting her die slowly of dehydration.
Farming friend says in all likelihood it was fluke, so I've learned a hard lesson about (a) dosing everything that's newly arrived and (b) asking for specifics when I'm told 'everything' has been done. What I didn't anticipate was how fast she would go downhill. When we dipped them on 8th October she was thinner than the rest, but not 'sharp intake of breath' thin. I kept an eye on her over the next week and she was grazing and running up the field with the rest and eating at the trough, but 7 days later when they were dosed, she'd lost a lot of condition, and the deterioration from Sunday, when she was still walking around, grazing and drinking, to last night, when she gave up, was very quick.
On a more positive note, limping lamb is feeling a lot better and farming friend helped me round up the other 10 lambs and give them all a trace minerals bolus. They're all eating well from the trough now. Limping lamb has another 3 days of injections, so will stay in the back garden for now and I'll see if I can get her a little tamer.
RIP little lamb and I'm sorry that I didn't do better by you.