We are great believers in Golden Hoof, which is a foot dip and I think is available from your agri store. We would dip every day for such a bad hoof if we had one, until it started to improve, plus antibiotic injection as stated above. Unless the hoof was in a terrible state we would wait to trim until it was starting to heal. To dip the foot we use a tin with some of the diluted mix in (currently a beer kit tin), sit the sheep on it's behind and hold the front foot in the tin for 2 mins. For a back foot, stick the sheep's head into a corner so it can't move, then stand the foot in the mix the same way. It really does work.
Incidentally the sniff test is quite good. Having smelt the smell of such a bad one, you will now recognise even a slight whiff, before things have gone bad. Mr F is frequently to be seen sniffing sheep feet
There is a case to be made for bringing the affected sheep indoors onto a concrete floor (with a lucky companion too) This way you keep the foot dry, and the animal isn't further contaminating your pasture. Once the foot is sorted out you can clear up the bedding and burn it, then disinfect the floor.