Mr Harrison was also contacted by a woman who works for an Essex-based firm which publishes magazines for the public sector, including one called Counter-Terror Business aimed at the police, the military and the intelligence services.
She offered to give him access to a database of over 100,000 email addresses, including potentially sensitive contacts within the Home Office and other government departments.
However, Mr Harrison said he became suspicious and contacted the woman's employers - PSI Ltd - to alert them to a potential breach of security.
One week later, the letter arrived at his home on the sparsely populated island of Sanday. One of the lines reads: "You dodged one bullet but keep checking over your shoulder, Jeremy is still watching you and is waiting.
"You see how we are working with the public sector to support Jeremy Bamber."