Thanks Soksta. I actually did see that thread in my search but somehow missed this line (think it may have been due to the way the original thread appears on my screen, quite messy!):

*17856 *013 * 0 or * 1 then #
Key Theft Protection - On/Off
NB: Only S4, 30, 32 & 34 models * 0 (off)

I guess I'll go try it and see how it goes!

UPDATE: Can confirm this works. Would just add that you need to put your PIN in before it will accept any *17856 codes, regardless of whether the car has been immobilised or not.