Our "Sticky" Dynamic IP has certainly changed somewhat, so it may well be the case that we have been shunted off onto another Card/Chassis.
I myself have no idea how often VMB re-issue IPs, but luke81 on page 17 of this Thread mentioned that someone from VMB said that it was weekly, this is what he said:
>> The dynamic IP holds pretty well, but it will change from time to time. I was told by customer services that the IPs get released "back to the wild" once a week, so as long as your router is turned on at that time each week, you should keep your IP. This also means that restarting the router should be fine as it will just grab the same IP as before, unless you are unlucky enough to restart at the exact moment they release the IPs! <<
As I say, I just do not know, but thought it may be useful to advise what happened to us today.
This was the first time the "Sticky" Dynamic IP changed on us since we had to dump the true Fixed IP because it did not work, and made the VMB 200 Line almost unusable.