[Mike]

That's an interesting one; I suspect that it was a simple case of switching the feeds at BT's end for the new franchises, so the outgoing feed from each new station was probably seen by BT (or whoever did switching for ITV) but not sent to transmitters until midnight on the 1st January, or 0600 in the case of GMTV.

I imagine everything after BT was the same as it was before, just going into BT was a different source rather than Thames/TVS/TSW/TV-am so a likelihood of technical failure was slim.

You can even see the physical change from sources on the Thames/Carlton handover that's on YouTube youtu.be 

Likewise I wonder how long Thames in this case, continued 'broadcasting' the ITN news before knocking it on the head.