Quick update - Now all 4 pc's that run office 2013 show the same issue, all after the silent update to version 15.0.4693.1002. tried an online repair, still have the same issue. If i do a system restore to prior to the update, excel works fine. another quirky thing is the excel worksheet is actually there just not displayed until you minimise excel then it is shown even if you subsequently go back to full screen.
I have spoken to office technical support who say they can fix this issue but only if i pay for the support !!! even though it is clearly an udpate caused issue.