I went for tests in the beginning of April and didn't hear for 20 working days so after much thought decided to post an email to the message centre. I was sent a reply to say I had passed the assessment day and to look out for an email inviting me to a unit visit and interviews.
My advice would be to email them on day 15 to stop the worry.
S
PS I have still not received an email about the next stage and that was 17th May.