bump,

any help at all, this thing is driving me nuts!

Give Messenger a call, and get him to put it on his T4 (testbook) as there should be some fault codes stored.

Check the wiring going into the inertia switch!. Easiest way is to remove the glovebox to get right into the wiring (4 torx screws at top, 3 hidden under mat in glovebox). Press the rubber button on top and you should feel a small straight gap, thats where the switch would pop up!. Now make sure the wiring connector is TIGHT.
I had the same symptoms and this was the culprit.....like you say, drives you nuts!