Just wanted to come back and thank you all for your advice. We ended up managing to book CR for the return journey:
QF BNE-SYD, then on to QF1 in F mid November, no bassinet for the SYD-SIN leg (5F 5K), but doing 4A 5A with a bassinet on the SIN-LHR leg.
Finnair from HEL-SIN in J late December...