Win10 PCs won't see ATtiny boards
There were a couple of comments that both led to the solution.

For me the most important was Nick's that the board pretends to have a USB interface for a couple of seconds, and then runs the sketch.  It stopped me tearing my hair out over why the board didn't show up as a port.

Ignacio suggested using Micronucleus.  I did this having installed the Digistump drivers and selecting them.

After compiling I was prompted to plug in the board, voila, it uploaded.

Thanks Stack Exchange!