Hi there (my first post so really hello!)
I too have an award booking, although thru BMI for LHR-BKK-SIN-MLE rtn in late Jan/early feb. I have had a similar problem, although it has indeed been an interesting experience using my award miles altogether!
Originally wanted to fly LHR-SIN-MLE but due to availability needed to book back in May for jan 09. Then, the woman at BMI FF line hung up on me after being rather insulting when I queried no availability on this route in any class (I wanted to travel in Bus), so when I complained they upgraded us from business to First, as long as we went TQ LHR-BKK First, SQ BKK_SIN Business, then SIN-MLE econ (SQ). This was replicated on the way back (and who am I to turn down two free F tickets on long haul?!)
But over the last two weeks SQ have sent me two sep txt messages, the last today, doing exactly the same thing to me as you - how am I supposed to make a connection that leaves two hours before I even touch down at SQ!?
I have to say that BMI were relatively helpful considering tehir normal standoffishness, and have moved our return flights to SIN-Munich in F, then Munich to LHR in Business. I have now called LH today to confirm and they were fantastic, reserving all our seats in all sectors for us, even though we are now travelling a grand total of three airlines!!! If our bags don't go missing at some point I will be very very impressed ;0
Have to say, LH service was fabulous today, and am looking forward to trying a new airline.
Don't despair, call the lovely people at LH - they will be fantastic and sort it all out in a minute!