If that's the same library which contained historical AIB/AAIB reports, I presume that it is now - hopefully in its entirety - lodged with the National Archives at Kew. Certainly that is what the AAIB intimated to me. If so, its only available to personal visitors, not online. The fact that the National Archives are in West London and I am in the West of France is the reason why I enquired as to the existence of an online source!