Thanks you all for all the replies.  I replaced this line in add_mpc.hee

^Click to download updated comet data and add it to Guide//dhttp://www.minorplanetcenter.org/iau/Ephemerides/Comets/Soft02Cmt.txt soft02cm.txt;dhttp://www.imcce.fr/fr/ephemerides/donnees/comets/ASCII/ELTDAT.TXT eltdat.txt;a2789^

with this line

^Click to download updated comet data and add it to Guide//dhttp://www.minorplanetcenter.org/iau/Ephemerides/Comets/Soft02Cmt.txt soft02cm.txt;a2789^

(i.e. I just edited out the ELTDAT.TXT part) and it works, although maybe I'm not downloading all the possible latest data.

George