I've had similar issues flying from LHR to Oz, and been deeply unimpressed when bassinet seats have been allocated randomly rather than to the only mother with baby in business class. The cabin crew have typically sorted this out, but its painful. I find EY disfunctional in respect of this kind of thing - BA (for example) allow passengers to book these seast in advance
For my next trip to Oz i was planning to book a seat for infant (who will be 18months at the time). Instead i have taken a First apartment for the both of us - see how that works out