What is the difference between 3amp and 13amp fuse?

If the fuse thsat has blown is a 3 amp, you should only replace it with a 3 amp fuse, which is good for appliances up to 700 watts (In the UK, on 240 volts). Do not replace a 3 amp fuse with a 13 amp fuse, it gives much less protection. If you replace a 13 amp fuse with a 3 amp fuse, the 3 amp will likely blow, since the 13 amp fuse should be protecting a powerful appliance such as a heater, between 2000 watts and 3000 watts.