[Update] As noted above, could not get a Micro configured with the built-in example "Slave Sender" to communicate with a UNO configured as Master Reader.
Today noticed that there are also Master Sender and Slave Receiver in the examples, so configured the Micro as Master Sender and the UNO as Slave Receiver and it did not work at first. Then removed the 4.7 k pullup resistor from the SCL line and it sprang to life!
Works as expected.