Thanks ieee48. I updated that thread with yet another library alternative, in case someone comes along later with a similar question. I'm using
which is quite current (Adafruit lib is about 2 yrs old now) and it seems to work OK so far. Author claims that his version of the SSD1306 lib is very miserly with resources and can be used on small controllers.
My original problem, amazingly, turned out to be the Arduino itself. Embarrassing. I have become a little complacent about my 'duini, I guess. I've never had one not work, even chinese cheapies. So I was trying out a new BBLeo (by Modern Devices, not a cheapie) and just assumed (oh dear) that it would work.
But there's something wrong with its i2c, not sure what yet; it will take a little time to figure out. Definitely flaky. When I swapped in a bog-standard Uno Genuino, all of a sudden I2C_Scanner started working reliably and presto, the OLED works too.
anyway my little blue OLED is now running through its demos happy as a clam. TTTT I'm not sure how useful a screen that tiny really is, but I couldn't resist them at the price!