Thanks very much for the help guys, much appreciated 
So, just taken a look at the module - unless I'm mistaken, it sadly doesn't look like it's possible to do NO operation with it. If you take a look at the PCB traces:
And compare with the datasheet:
Pins 1 and 3 of the relay are hard-wired to the outermost contacts of the module, and the inner most contacts on the module are hard-wired to each other. Here's the view from the top for comparison:
As I understand it, switching the relay on swings the connection from pin 3 to pin 4 of the relay, opening it. It doesn't look like pin 4 is connected to anything to me...
Unless I'm talking nonsense - there's a strong possibility of that, mind.. 
If this is the case, I'll rewrite the logic of the Arduino code to send the output "low" when I want to switch the air freshener on - annoying to have power blips on start up, but it'll have to do for now..
EDIT: In fact, I was talking nonsense about 'power blips' - have the appropriate pin set to high in setup and it seems to be seamless! Lovely.