The Web Interface for NIMS has comprehensive  help built in, so I'll just point out the salient features...

• The Web E-mail Interface is currently accessed via the URL  https://mail.theplateau.co.uk:8484 which gives full secure access to mail.  It'll prompt for your username and password before displaying any information (which is unlike the old Plateau Web interface).  When you visit the site it may give you a dialog box concerning the it's certificate.  Accept this, this is the mechanism used to protected data travelling between your computer and the e-mail server from prying eyes.  If you want to get rid of the message visit the E-mail Security page which shows you how.  
  IMPORTANT:  Please note that the URL starts "https://" rather than "http://" - "s" is for secure.  If you have a very old browser which can't deal with a secure system, then mail can be accessed via http://mail.theplateau.co.uk:8383 - but you won't be able change your password!
• As mentioned you can change your password via the preferences page - this the same password as you use for all other access to "The Plateau" - i.e. POP3/IMAP mail, FTP and Web.
• The Web Interface supports the usual web interface features - folders, address book, signatures, forwarding, vacation messages etc (don't use an "=" in the signture unless you can speak French - interesting bug!). 
• The Web Interface supports a POP3/IMAP proxy.  This means you can configure it to collect e-mail from other e-mail systems (with the exception of Hotmail/Yahoo/etc - which don't support POP3) and then read it through NIMS.  For example, mine is configured to collect my BT Internet e-mail.  All this means I only have web site to visit for my e-mail while away from home.
• The Web Interface supports e-mail rules, much like the common e-mail clients (Pegasus, Outlook Express, etc), which you can use to shuffle your mail into folders, delete known junk mail etc.