One of the simplest and most popular options, fan heaters work on the simple principle of using a fan to blow air across a heating element. This air then moves around the space, gradually heating up your whole marquee. Fan heaters are cheaper to buy or hire than most other options as they are relatively simple pieces of equipment, however they have relatively high running costs as they tend to use a significant amount of electricity.
Traditional convection heating works by heating up a small part of a larger space, then waiting for the warm air to move around, slowly heating the whole space. Radiant heaters are designed to provide direct heating to the area around them, with patio heaters being of the most common examples. Radiant heating is best used if your marquee is going to be partially open, as it avoids the issue of all the heat you generate escaping. Instead they provide a constant comfortable temperature to the area immediately around them, although this does make them less efficient for heating larger spaces.
These are particular type of radiant heater, which use a filament that emits infrared light to transfer heat energy directly to the surrounding air. This is a highly efficient way of heating a space as over 80% of the energy used by the heater is converted into heat energy. Infrared heating is therefore one of the most cost effective options for heating a marquee. Halogen heaters are probably the best known examples of this type of heating.