if the ticket is ex-UK (starts from the UK and priced in £) then you have to ring the UK office. SIN will not be able to make changes. All due to different ticket conditions issued by different tickets. Not sure if this applies to full fare tickets.
When I wanted to make a change to an ex-KUL ticket (its much cheaper than ex-SIN and ex-UK tickets), I had to ring the KUL office.
Could try the Ticketing number.
http://www.singaporeair.com/saa/en_U...fo/contact.jsp
Good Luck.