I tried to get in the habit of pushing the ASS "off" switch every time I started the car, but I must have forgotten at least 50% of the time. I've never got the hang of pressing the brake pedal just enough to hold the car without the ASS activation - sometimes I could, sometimes I couldn't. 
Several people (here, and on PistonHeads) told me to buy software and code it myself. After the grief and expense I had trying and failing to code my E46, that was never going to be an option.
I was pointed in the direction of a local(ish) lad by another member on PistonHeads. Initially, Craig-the-coder thought there might be an issue because my car hasn't had a 2013 firmware update - it's actually running 07 2012 firmware. As I understand it this earlier firmware doesn't allow full deactivation of ASS like the later one does. What Craig has enabled for me is the function for the car to memorise the last ASS setting when the ignition is switched off. The whole process took about 10 minutes, and Craig charged me £20.
If last winter - when the temperature was low enough that the ASS rarely activated itself - is anything to go by, the increase in fuel consumption will be negligible.
I'm happy.
