Had the same problem - really irritating. Suffered for a whole year!
Look under bonnet -impact sensor on left with red rubber on top. Press rubber button with keys not in ignition - should re-arm -if not remove red cap and check that when pressing to re-set the mechanism is fully pushed down.
Very easy to replace from a scrap yard with no need to reset onboard computer if like me the button just wasn't pressing down enough. Just two pins and the plug underneath just pulls off.
I'm gonna miss those flashing hazard lights!!! NOT.
Wendy
April 2010