What size fuse should be used in my appliances plug?
There is a formula for working out the fuse rating for each appliance.
P (Power in Watts) / V (Voltage) = I (current in Amps)
Note that you should always ensure that you round it up if it is not a whole number.
So, for example assuming the Power of your heater is 1200W and the UK mains voltage is 230V:
1200 / 230 = 5.217
You will therefore require a minimum of a 6 Amp fuse in your plug.