Thanks for your answer Richard.

When I called UA back to complain about the missing segment I tried giving them the Amadeus reservation code so they could see the problem for themselves, but the agent said she could not locate any reservation under that number. I assumed this had something to do with UA maybe not having access to Amadeus as my initial rebooking had been made solely on the basis on my name and flight number.

I this is indeed the case and UA can only see my UA flights I seem to cought in somewhat of a tangle here as I guess no travel agent would be able to locate the missing segment either. Any ideas on who to call? If UA's US offices are more experienced with RTWs than Singapore I'm thinking they may be the best to call.