Hi,
Just a note that under UK Law (construction and use regulations), it is illegal to have the engine of a car running if the driver is not in the car. The only exception to this is were the engine must run to power some auxillary work device such as a compressor on a recovery truck.
This law also affects the engine stop timers that some people fit to turbocharged cars to let the engine cool at idle. As has deen pointed out, by others, your insurance company may take a very dim view of such a device. And we all know that they will use any excuse not to pay out!

Robert G8RPI.