HMRC and government in general could do worse than take a 5-minute tour of the US's IRS e-filing website. This is a really easy way to work through the messages which are clear, unambiguous and comforting. There is one tiny usability issue. You have to know that you click on the Individual, Business or Tax Preparers buttons in the left hand pane.
Who fancies letting the suits in Whitehall know about this?