Hmmmmmmm... well first off I would find out if they want a dialect... and are you comfy with a Brit dialect? There's also a need for a South African dialect in that show, I believe. IF they say yes and you aren't comfortable doing one, you might just honestly say that it's something you believe you could acquire but just need time to hone it and for the moment would just like to read straight.
Those Agatha Christie plays are wordy as hell so give yourself as much time as possible to study whatever it is that they want you to read. Get there early AND get yourself somewhere that you can read it out loud, DON'T try to go into the audition after having only read the sides silently.
Um... just remember that if you suck the won't shoot you and that they want you to be the perfect choice as much as you wanna be.
BREAK A LEG!!!
yer palâ„¢,
VOLveeta