I've finally figured this out, it's actually very easy but finding it in the vb code was a nightmare!

Open ~\admin\Users\ManageUsers.ascx.vb
Go to line 268 where it says:

If Not Page.IsPostBack Then
      PageNo = 0
End If

Change the '0' to whichever panel you want to display from this list:

Dim showUser As Boolean = (PageNo = 0)
Dim showRoles As Boolean = (PageNo = 1)
Dim showPassword As Boolean = (PageNo = 2)
Dim showProfile As Boolean = (PageNo = 3)
Dim showServices As Boolean = (PageNo = 4)

Hope this helps!
Matt