Yes it worked, using as you suggested:
=IF(B45="YES",(B8*B9*0.5)&" Kilos",0&" Kilos")
But i tried to get IF NO = 0 kilos using:
=IF(B46="YES", B9*B8*0.5,IF(B46="NO",0,"Not yes or no"))&" Kilos",0&" Kilos")
Where have i gone wrong??