Actually, to go a bit against the grain here, maggots are not so totally bad in a foot, unless they spread to the rest of the sheep. They will clean out a foot with footrot (just as they can clean out infection in a wound, but as soon as they've done that job you get rid of them before they start on healthy flesh), so as long as you discover them and spray with Crovect, they will have helped the situation not hindered. Then you can treat any remaining infection in the foot.
Maggots give most folk the heaves, but even they have a good side