Ivermectin may help kill maggots on the sheep if they are eating the flesh, as they will be taking in the drug that is in the sheep's system. It is totally ineffective if the maggots have not started eating the sheep.
You need a pour on topical treatment, either Crovect (or equivalent product as crovect is hard to come by as stocks are low due to a really high demand this year) or Coopers Spot On.
You can certainly cover the sheep with antibiotics so that any broken skin won't get infected with bacteria as well, but please get something to kill any maggots and eggs that have crawled away under the fleece too. Some blue spray will at least give you a bit of time to get away to get some treatment for her.
I would make sure to preventatively treat the other sheep as well, give them all a good thorough going over. Crovect can be used both to treat and to prevent, so well worth asking for some of this from your vet, or a local farm supply shop, who sound like they will be a lot more helpful than your vets!!
Good luck