It looks like I spoke too soon - my ip address was gone again this morning.
Support wanted to swap out the cablemodem again, and I didn't have any other ideas, so this pm an engineer swapped my Ambit 200 for one of the new Ambit 250 vertical modems.
He was very helpful and said that if the issue happened again, then after being swapped twice, it almost certainly couldn't be a CM issue, which at least narrows things down a little more.
However we had a discussion about how the DHCP ip address allocation process works - and it turns out that the UBR supplies my ntl ip address, and that ntl record every single DHCP ip lease that is given out, and it might be useful to enquire about this on any future failures, since that could point towards a problem ...