Then why move it from a slot where its relatively low ratings are par for the course to a slot where its ratings will be under far more scrutiny?
Because being out-rated nearly 10:1 by Marr on BBC One an hour earlier, means those ratings aren't 'par for the course'.
"par for the course". The ratings in the Sunday slot are near identical to the repeats previously shown in that slot, but probably a more attractive audience to advertisers (hence increasing ad minutes in the slot). The network's losses are minimised.
Slot averages are just that - averages. They aren't expected audiences for a show. Some shows massively outperform their slot. Some underperform. I wouldn't call hitting a slot average 'par for the course' for a show. If Strictly got the slot average ratings on BBC One on a Saturday, would you call that 'par for the course' for Strictly?
Peston seems to hit the slot average you say. However that average is not for a high-profile signing presenting a live show. Not outperforming the slot and being massively out-rated by a rival show in a slot only an hour earlier must surely mean that the show is not seen as an unmitigated success.
The programme is also attracting the right guests despite the ratings and is getting headlines to build ITV's political cachet. Until this announcement, ratings had barely been mentioned in the press following the launch, so the crap numbers weren't visibly hurting ITV's reputation.
I suspect that behind the scenes the Peston team were having to work harder to avoid being the 'also rans' in guest booking terms though. Some weeks there are more credible guests than outlets, and some outlets work better to some interviewees strengths than others. However if you wanted to make a big announcement or provide a big interview, you'd have been crazy to have chosen Peston over Marr if you wanted to reach an audience.
If - as you suggest - the network thinks it's a stinker that turns off viewers and it is being kept purely out of a sense of duty / reputation building, 22.35 is a completely mad slot to move it to. If it rates anywhere near as poorly, ITV will lose money on the ad slots and - as you say - they won't get the same headlines / brand cachet out of it because of the timing.
I didn't say that - just that being out-rated nearly 10:1 in a very similar slot after a sensible length of time to bed in, means it's not proving to be an audience success. Peston is obviously talented, and has his own audience. Maybe they think that audience is more likely to watch on a Wednesday than a Sunday, particularly if that audience skews younger.
Could also be that there are other reasons - Robert may not want to be 'up against' Marr/Sunday Politics all the time. He may want the show to do something different.
This is a perfectly reasonable approach if the next step is, in six months, to say that the programme isn't working and is being axed. It's also a perfectly reasonable approach if they genuinely think they can build the show this way and make a success of it (maybe - as you suggest - by morphing into more of "This Week" than "Marr"). It's not rational if the intention is to maintain it for the long-term as a tool to convince people of the network's merits as a PSB.
No - put a PSB show that nobody watches isn't that useful either.