Ok now I get it! I have added the format to the entercell function, and added the number format, and every thing work well.
The issue was in the original excel template some of the cells were formatted to show "as thousands" ie 1,000, now the entercell function takes care of this.
Once again many thanks. I now have an Nav 2013 working excel cost sheet to keep my users happy.
Let's hope the actual system works as well as the report when we go live !