It seems to me that there is currently a pricing glitch where for some reason the differential between a F fare and an R fare isn't pricing properly across the major GDS systems. This causes the fare to break in SIN (resulting in the LHR-SYD F fare being added to the SIN-SYD R fare). What should happen is that the LHR-SYD F through fare should be first calculated, and then the difference between the SIN-SYD F and R fares added to it. (Or maybe I am wrong and SQ really wants to price fares like that?)

Try booking a flight from LHR-SIN that only has C class and combining it with a SIN-SYD flight with R class and the differential thing works. (LHR-SYD through fare in C calculates with difference in C/R fare SIN-SYD added on)

Some flights on the SQ A380 in 2008 are still showing as having "F" class available on the GDS systems, but come back as "uncomfirmable" when a booking is attempted.