Rob Green, who has long maintained his aunt was not the victim of a bungled burglary but killed because of her anti-nuclear work, says the 30-year rule on disclosure of confidential government papers may lead to documents about Miss Murrell's death in 1984 being made public.
The former naval commander has travelled from his home in New Zealand to Shropshire to discuss the British publication of his 2011 book, A Thorn in Their Side, and to attend a pilgrimage to a cairn – a memorial of stones built in Miss Murrell's memory near Llanrhaeadr, one of her favourite places.
Mr Green said: "We returned to Shropshire in March, the anniversary of her death, and had been hoping to join others on the walk then. But it had to be postponed because of the snow."
He said he was finalising the text of his book for UK publication.
"It is all about the pursuit of the truth about my aunt's death," he said.
Miss Murrell's body was found in a field three days after her disappearance from her home in Shrewsbury in March 1984.
In 2005 Andrew George was convicted of her kidnap and murder and sentenced to life imprisonment.
Mr Green said: "There is a man in jail convicted of her murder who I believe should have been acquitted.
"He was in her house, yes, but he was a 15-year-old who could not drive. I believe he is too scared to say what really happened.
"I hope that the 30th anniversary of her death next year will see questions answered when papers are released under the 30-year official secrets rule."
More than a dozen people including Mr Green and his wife, Kate, walked to the cairn to lay a stone on Saturday.
They included 73-year-old Daphne Phillips who was a friend of the rose grower and naturalist who said: "She played a huge part in the work of the Council for the Protection of Rural England and Shropshire Wildlife Trust."
Mr Green said it was wonderful that his aunt's memory was being kept alive.