Biz knowledge, know your perpetuals from your zero coupons!

They'll check that you really know any language to claim to know with
pretty careful and well-designed tests. Thats a screening test for a
phone interview with a higher-up.

Their systems seem to be pretty much unlike anybody else's, so they
didn't seem worried about business knowledge: about the only other gang
in the same business is Reuters. They do expect you to understand
writing efficient code, the prod and cons of multithreading and the
reasoning behind the choices you made when designing any systems you
claim to have designed in your CV.