Are you sure it's nappy rash and not thrush? The way you describe 'spots' sounds a lot like a fungal infection. You need to use an antifungal cream at every nappy change until several days after the rash clears if it is thrush. Ask your pharmacist, I used one which starts with B... anyone??
This is a good point. A thrushy rash will have isolated spots away from the main rash and tends to start as spots as well. It would be a good idea to treat her orally as well. Either nystatin (nilstat) or daktozin are good. Both come as an oral and topical preperation and available over the counter at chemists.