I answered my own question - rather unexpectedly.
I went into song edit and deleted the first 15 bars since it wasn't necessary. Then when the keyboard asked me to save, I renamed the file and inserted the floppy into the drive. The keyboard picked up the floppy asked me whether to save there, which It thn did without any hitch. No internal memory full message or anything.!!!

Very weird. A result hoped for but very unexpected.
Thanks everyone
Alex