Red letter day?

Both last year and again more recently HMRC have been sending out letters to tens of thousands of contractors and subcontractors in the construction industry. These letters are concerned with the employment status of the workers engaged and give the impression to the recipients that the game is up. Most practitioners if not all will have clients who have received such missives. Should they and their clients be worried? What action should they take?
The first thing to note is that all such letters are computer generated and have not actually come from a person at all. None of the letters is signed although it is good to see that in this year's version of the letter HMRC appreciate the difference between 'yours sincerely' and 'yours faithfully' and last year's letters were not even dated. The current batch of letters have a month...