I flew this route in April this year. I used avois to upgrade SIN-SYD from WTP to CW and then SYD -LHR again from WTP to CW (sadly no availability for LHR -SIN but WTP was fine). During boarding at SIN (outbound) I had the magic beep at the gate and was upgraded to First. is it possibly more likely then for an upgrade on one sector rather than the whole journey? Or is it that the likelihood increases if you split the journey? I'm not sure.