Yes, you have that right, MalcomC. The vignette is a well-known characteristic “feature” of this lens and it is supposed to be that way. Everyone who has reviewed it has pointed this out. As Dustin Abbot put it . . .
" . . .what I saw was clearly a mechanical vignette – an obstruction that kept the corners of the frame from receiving light at all [as Perfrode correctly pointed out]. This was one of the compromises that Canon had to make to make this lens work. […] The lens actually frames wider than the physical design allows for, and then, when corrected, it shows the proper look at 24mm. […] You can see just how much latitude that Canon is giving itself for digital correction. One just has to accept that this is part of the len’s design, in that it was clearly not possible for them to go all the way to 24mm without something having to give." https://dustinabbott.net/2020/04/canon-rf-24-240mm-f4-6-3-is-review/
It’s up to your taste actually. But as @Joanna already pointed you can’t modify the default preset. You can create a copy of it and modify or take the one suggested above.
@Joanna @sgospodarenko
Thank you both for you replies. I hadn’t appreciated that you can’t edit the default presets. I’ve already installed the preset that Joanna posted and it works perfectly.
Some people can live with it, some not. I think that Canon however should be honest about this. It should say on the Canon pages and be informed about by the sellers. “At 24 mm on a full frame camera, some vignetting may be present that may be difficult to correct even in post processing”.
The lens is actually wider than 24mm and is designed to be used with software to get rid of the “vignetting”. As I’m sure you’re aware, software correction is widely used these days, especially with more budget lenses. I believe that even after it’s been cropped, the result is still wider than 24mm.
Good luck with your back. I hope you get well! 
Some people can live with it, some not. I think that Canon however should be honest about this. It should say on the Canon pages and be informed about by the sellers. “At 24 mm on a full frame camera, some vignetting may be present that may be difficult to correct even in post processing”.