Thanks Erwan, brilliant, solved!
When the board alias "spi-1 = &spi1;" was added the executable built ok, no errors!
Note that an underscore in "spi_1 = &spi1;" threw an error: dtlib.DTError: /aliases: alias property name 'spi_1' should include only characters from [0-9a-z-]
But then it's clear from the documentation that dashes in the .dts become underscores in the source.
Am not bothered about the actual hardware at this stage, as the idea is to be able to use an example that builds ok as a template to be modified.