OK, multi-city sort of works for me now. What I was trying to do was:

2 Dec LHR-SIN
5 Dec SIN-BKK
11 Dec BKK-SIN-LHR

Now, on the 11 Dec, I was trying to get the early morning connecting dayflight (SQ61 and SQ320) which was OK on the old system, but this is what the new booking engine cannot find a fare for (or any combination of BKK-SIN-LHR fares on the 11 Dec).

However, if I do the BKK-SIN on the day before, i.e. 10 Dec, and keep the LHR-SIN dayflight on the 11 Dec, then it prices out perfectly.