Billy / Les - hi,
Singapore airlines are not too bad, although they don't match up to the standard on which they built their reputation - although their flights are nearly always full! Legroom is nothing to shout home about-only slightly more than BA. (we like to try for the two window seats at the rear of the plane on the LHR to SIN , and return, legs rows 61 to 64).
Many people say that Malaysian Airlines are better but we enjoy a few days in Singapore.
Les -I've not tried the route via Doha but I seem to recall that the return leg from DPS had a longer wait than the outbound leg, making for more than 24 hrs total. The SIA flights are a bit tight, but if you book straight thru from DPS there is about 50 mins only in Singapore on the return flight meaning about 17 hrs overall.
Qatar are offering remarkably good prices and get good reviews
http://www.airlinequality.com/Forum/qatar.htm - Enjoy - it will be worth the wait.
Peter