Thanks for your answer. I found it.
I now ticked "Consolas" and it has the desired visual difference between "one" and lower-case-"L".
I think i had a GUI understanding problem - the ribbon buttons in Diff/Merge were not serving the purpose
and the small drop down menu (for ribbon customisation) did not offer any settings (or maybe i just overlooked such a button option).
The very top-left embedded icon was never clicked by me as i understood it as /decoration/ (such as the Firefox title bar could be decarted by "personas")
but in fact its a _variant_ of the "File" menu with open options and even the option for generation of diff files (yes, i never saw that...)