here is a possible solution for you. it is originally written in french so the second line is a translation and might have to be checked in your help file.
=SI(ESTNUM(TROUVE(",";A4));(STXT(A4;(TROUVE(",";A4)+1);5)+(TRONQUE(A4)*14));(A4)*14)
=if(isnumber(find(".",A4)),(mid(A4,(find(".",A4)+1),5)+(trunc(A4)*14)),(A4)*14)
the above assumes A4 is the cell in which the stones and pounds are in.
also, i believe that the english version of excel uses commas as separators in place of my semi-colons.
pls advise if this helps