Thanks for all your help on this one - today we were about to book our flight Sin-Per and decided to call the FF consultant first to check about the baggage LHR-Sin-Per - the fact that we would be flying J from LHR-Sin and then have to purchase economy Sin-Per on another reference. We had enough FF but didn't want to use 100,000 to fly back in J ...
However the consultant said we could add the Sin-Per leg to our original booking reference and we would be charged the lesser FF as per booking LHR-Sin-Per in one transaction - used 63,000 points instead and ended up on the one booking reference so can go with the earlier flight and baggage all the way home. Also eliminates the problem of flying J all the way except Sin-Per which could end up a problem with baggage weights and paying excess.