Thank you for your help. It is quite complicated to work out but going through the site you stated and ignoring the Omnibus editions and the non-crime novels, it looks as though the answer is 79 - but I could be wrong!
back when I collected them all ,I got to 77-thats if you do not include crime stories that are collections( The funny thing I always thought is that she always said she preferred writing the romantic rubbish(!) and yet never liked writing the romantic part into a crime novel??)