Originally Posted by PLeblond

You purchased a trip from between SIN-LIS not individual segments. If you call in to change the outbound they will reprice the ticket to SIN-LIS and LHR-SIN which will probably cost several thousands of dollars more.

As stated above, if you miss any leg, it automatically cancels all other legs on the ticket. Some people chose to skip the last leg, but I consider that morally wrong when done in an intentional and premeditated fashion, (others without character will certainly chime in objecting to this opinon, but I digress).