Additional info, pxemtftp.log contains this message:
' Tue, 26 Mar 2013 13:58:43 PxeMtftp: Unable to bind to socket on mulitcast port 57094, Error=0x2740'
Found simalar thread from other vendors but they point to anothe TFTP server running on same subnet. Which is not in this case and two othe units Dell Latitude E6430 and Percision Laptop M4400 work fine.
Regards