Who or what is the source of this letter?
Did Robert Boutflour, Jnr., confirm he was the author of the letter?
Can an image of the original letter be provided that manifestly matches the transcript?
Was the letter adduced in evidence at the 1986 trial (albeit, it was not material evidence, so probably not seen by the jury)?
Probably it is genuine, as there would seem to me no reason to make up something like this. Even if Robert Boutflour, Jnr. - "Uncle Bobby" - was plotting behind Jeremy's back, he may still write a polite letter of condolence for Jeremy and send it. Just normal, considerate behaviour, really. Furthermore, if Robert believed Jeremy to be guilty, why shouldn't he plot behind Jeremy's back? If Jeremy really is a mass murderer and double child killer, then I'm really rather pleased there was a conspiracy against Jeremy that brought him to justice. But note the 'If'.