OK, so Sally's theory worked. I went out at half 4 and she no longer wanted to get up. Popped a halter on her, injected her, sprayed her foot again. The small split that was there yesterday has completely gone but her foot is slightly warmer than the other one.
All good apart from the damn syringes. I have injected before and never had such bother. Both the needles came off the syringes and had to be removed from the (non-plussed) ewe. The painkiller one came off before the last bit of the medicine had gone in and it sprayed in my face (Loxicom does not taste nice!). I had checked, as I always do, that the needles were securely on the syringes before I used them

Hey ho. Job done and now just fingers crossed that she'll be on the mend.