Have you tried File--document presets --define
Thanks for the suggestion. I have tried that but unfortunately the problem seems to be that the offending preset is now classed as being a default preset which one cannot delete going the file-document presets-define route. It will only allow you to highlight (and therefore delete) custom presets that have been correctly created, saved and titled. I mistakenly didn’t do this (not entirely sure how, but academic, seeing as it’s happened) so it was saved under default presets. Any further insight most welcome!
Why don't you just change the default to what it was before, save it and create a new custom as needed?
I seem to have resolved it by doing file-presets-default, selecting preset and then deleting. I'm still not entirely sure how, but anyway I got the desired outcome! Thanks for all advice offered.