My local pub now has the outdoor heaters on a time switch, i think they stay on for about 10 minutes.
The switch is an IP rated one though not sure of the make, however it is next to an IP rated enclosure so dont know whats in there, maybe a contactor etc.
Also as an idea, what about a coin operated meter for the customers to use if they want to stay outside and have heating as well?
It might not be a popular decision but at least it will definately reduce the pubs overheads!