According to KVStool there is plenty of D-class availability (using Amadeus or WSpan). There will be few exceptions, e.g. on the 19th itself BA16 shows D0. Pricing the entire trip via BA10 return (leaving somewhere mid-December) comes up with a 3800 euro fare and books in D-class (with POS=AMS). So I'd try to call again and ask the same....

BTW: I assumed you are travelling AMS-LHR-SYD.