I'm not so worried about the incomer, since UKPN will fit a 100A fuse on a 16mm2 incomer (and as you say they may be larger). But they won't for 16mm2 tails, hence my wish to get them upgraded if they are in fact 16mm2.
If the incomer is OK then I doubt that the DNO will replace it, they may change the head if they feel it needs doing. I doubt very much that the incomer is anywhere near as small as 16mm, because ever since the year dot incoming cables have been greater in current carrying capacity than the tails, not least because the fuse protecting that cable will most probably be rated at 800 A.
I had a reply from Octopus saying their currently install time is two weeks, which is good. They didn't answer my question about isolators and tails, so I asked it again. Let's see if it takes another 2 days for a reply...
A quick update - I emailed [email protected] and they were more responsive than the regular [email protected]. They said that installing an isolator and upgrading tails would be free as part of a smart meter installation, just to let them know at the time of booking. So I will start the process to switch to Octopus. Result