I just looked on EF and it appears BA 16, SYD-SIN shows D9 to SIN on 1 November, then D0 to LHR, while QF 1 shows the opposite (D0 to SIN, D9 to LHR.) The connection time at SIN is roughly 2 1/2 hours. I think you're running into a married segment problem. I'd phone the AA desk (if that's who you're using) and see if they can book it as two segments rather than one married segment. It will cost you a coupon, of course.