Hi CB,
You are correct that the Silverjet cabin crew wear an open shirt. However, when I went to the selection last month I wanted to avoid dressing like I was already an employee. I was under the impression that the male cabin crew wore a silver tie and a white shirt (like the pilots do I believe), but this is not the case. Imagine my surprise when I realised that all the male interviewers were wearing a blue and white horizontally striped shirt - the same as I was! Doh!
Anyway sorry about going off subject slightly. All the males that were there for selection when I was there had ties on so you might feel a bit too casual if it is the same when you go, even though the interviewers will not have a tie.