Let’s be clear - Hendy wasn’t doing it in an underhand way - it was a letter on headed notepaper. If he wanted to do it without publicity, he would rung the chairman of Systra and not put it in writing at all.
Spelling out the consequences of any public statements or accusations by either the supplier or one of its employees, especially a prominent employee, is not unknown. In a public contract you usually have an agreement that you cannot, either directly or implicitly, slag the client off or act against the interests of the client. If you are an employee you do not have the right, as a private citizen, to just go to a journo and spout off about one of your firms clients and not face any consequences.
I don’t think people here realise how organisations (both public and private) work at times. Pressure is a recognised tool of the trade and is often deployed. It can get threatening if one party thinks they have been wronged and those at a senior level tend to have notoriously thin skins. The DfT are another party who have, in the past, “made changes”, on demand of senior officials.
Like I said before, it’s a hard world out there and if you mess with the big boys, you are likely to get in trouble if you are not careful. Blur the distinction between your professional and private lives and it may well end in tears.