Hello Ian,
I am not familiar in detail with the heater you refer to, but if LMH's descriptin is correct then I have to agree with him - it is a potential fire hazard, not to mention the considerable gas usage that would ensue. - It is quite possible that Butane bottles would be totaly useless, and you wodl need an industrial sized propane regultaor to cope with the gas flow. These heaters also tend to make a considerable noise when runing
One of the biggest problems with heating air in what is basically a canvas tent, is that there are lots of holes where the hot air can escape, and the fact that canvas and nylon are not good heat insulators means that the heat easily escapes and is wasted.
Considering your need - to heat people in the awning - there is a possible alternatives, that may be simpler and much more effective. - Infra red- These heaters transmit heat via Infra red radiation. This travels in striaght lines, so to feel the heat you must be able to see the heater.
These heaters tend to be directional and as heat is only produced where the radiation strikes a surface so less will be lost.
There are several electric versions, in thier most recent for known as Quartz or halogen heaters, but there are some versions that use gas, but these are mcuh rarer and again for safety reasons may not be suitable for awnings or tents.