I am trying to book LHR-SYD-LHR on BA 11/12 LHR-SIN and 15/16 SIN-SYD and ba.com is saying there is no availabilty in that class on the 15/16. Yet if you book right through LHR-SYD on the 15/16 (at the same price) the fare is displayed in WTP the whole way.

Edit - Problem Solved
Having spoken to a helpful BA Silver Sales agent, the reason the website was doing this is that T class is available on the first sector and not in the second, which only has E available. The website then defaults to the cheapest option with T which is T for the first sector and economy for the second However if you click on the small link for 'more flight options' the same flight then the same flights appear booked in WTP in both sectors for a higher fare - however without any pricing on that page in a much older style of booking interface from a few years back. I guess the fare structure is too complex to display in the current standard format. Something new learned today, but I don't think this functionality on the website is very intuitive!