Yes Andy I am making progress. I am using astropy on Windows in a very amateur manner.
Nonetheless,I have managed to extract the first of the test spectra from the multispec format and saved it in a form that ISIS will open and display correctly. It helped when I opened the .FITS file in ‘update’ mode then I could add the extra header data. Just need to put it in a loop to get all the orders out and tidy up the header data. (Tim Lester’s PlotSpectra currently objects to the date and time format!) I will need to read the BSS spec carefully.