The problem has now gone away, leading to one of those "go figure" moments.
All I did was edit the original file with preamble to include 20 G1 movements in X only before it started. All of them executed. I then edited out only what I had put in and everything now works.
Somewhere between creating the file in LibreOffice Calc, saving it as a .csv, editing it with notepad, loading it into Repetier Host and saving it to SD some weird magic occured - but the good witch must have come along and made it better.
Mike