A few people have asked about the letters and my opinion on them. There are several schools of thought. The letters could be genuine and sent from the killer, but could be just guesswork and sent by a random person like the Humble letters in the Yorkshire Ripper enquiry.
Readman, the SIO at the beginning of the case, stated that they were vivid and compelling. I have identified some links to Benson, stated in open-source material, too. However, there is nothing to say either way.
Numerous people were asked to give handwriting samples, including Peter Heron, and no one matched.