OK, I understand now....apologies for being dense. I had (incorrectly) assumed that the "set" function would only work for a given file type (nef, raf etc). Having set it for nef I see that it is available for all raw files (that I have tried so far).
I had assumed there would have been an option to select other raw converters and got confused by seeing the list of file types.
Thanks for sorting me out.
Dave