Hmm, it's a bit of an unusual one because they are both two very long-hauls and the particular combination of F and Y is not too common. I don't know the 'correct' answer, but I suspect at least one of the lounges at SIN would let you in. I would try QF F first. If that's a no try BA and if that's a no try QF J. I wouldn't expect QR to let you in but who knows.