Hi,
I heard that the interviews will be held both at Heathrow and Frankfurt, basically you should expect a maths test, language test, and a group discussion (but I don't really think the japanese are very interested in the progress of Big Brother or in testing your singing skills ...I personally hate this type of tests as well...
)
After that most of the people will pass to the personal interview with usually two interviewers, well this was my assesment day for ANA. I personally found the assesment day to be quite relaxed compared with other airlines, as they usually invite small numbers of candidates on each day so you don't feel the competition that much...
take care