Is your LHR-SIN-NRT one award?
If so, you have a Zone 12 (LHR) to Zone 7 (NRT) Standard award confirmed, and a Zone 1 (SIN) to Zone 7 (NRT) Saver award now confirmed?
Unless you want to change your LHR-SIN-NRT Standard award to LHR-SIN and have a separate SIN-NRT Saver award, I don't think you can do anything.
Did you waitlist instead LHR-SIN-NRT at saver level?