I do understand the fault code would still be in the ecu ,but surely the error light on the dash would go out once the defective part was replaced even though the error was not cleared out of the ecu....wouldn't it ?

I actually disocnnected the old unit and put the new one whilst the battery was still connected. Obviously the ignition was off. Surely that's just one of those things they always advise you to disconnect the car battery , but really you can get away without disconnecting. Anything you do on cars, however minor they always tell you to disconnect the battery ! I mean if you replace a fuse they don't get you to disconnect the battery ? So in this example what's the difference between pulling the airbag fuse out with the battery connected ? isn't it the same thing ?

I only disconnected the battery after the second unit was still showing the fault led on on the dash with the ignition on. I thought it might clear/reset something - although I do understand the error would still remain in the ecu.