If you're using a marquee during the winter months then you will need some heating. Even if you're using marquees during May or September then you may need some small heaters just to take the chill off later in the evening.
Heaters used in larger marquees (wider than 3m) require a fan. Cabinet heaters, halogen heaters and small electric heaters are all of limited use in a large marquee as they do not cover a large enough area.
Some tips for marquee heating: