@volunteerpilot & all
I have just had my assessment and I have to let you all know that the info which appears on the BA site, I believe, is correct. There will be two further stages after the FTOs, both taking place at Waterside. One of them at the end of April and one at the beginning of May.
I am not sure, however, why this will happen and what exactly can they possibly ask of you in two interview days.