I struggled to find any info on Apple Support as to why my search wasn't working.
I can't find a free version of FindAnyFile.
I found this video on YT:
https://www.youtube.com/watch?v=v1cBhIRROAg
[Although the "Raw search" part was beyond my scope and needs]
It seems that cmd+f is the way forward. Using the modifiers it finds all the files containing "257", which the standard Finder search couldn't.
[I still don't understand why the standard search doesn't return files in the results that it knows are there, but I've got to let that go.]
Hope this helps any other Windows converts who might be struggling.
I am now going to try to learn how to set-up Safari. I don't want to take the easy way out of downloading Chrome without at least trying, but Apple aren't making it easy.
Edit:
After learning of the cmd+f keyboard shortcut I searched for whether there was a mouse click for this. The consensus seemed to be that there wasn't.
But if you right click on finder in the dock you get an option 'Find', which brings up the cmd+f window where you can search with the modifiers.