Sion had contacted Sussex Police over the puzzle that his original witness statement written at the time said he had spoken to a plain-clothes police officer in the hallway of his Lower Park Road.

However, no witness statement for that "officer" exists and he now believes it may have been the killer.

The Observer previously reported on Sion's claims and published an e-fit of the man he produced with The News of the World.

At the time police said they were considering his concerns.

This week they gave this response: "We have identified, to our satisfaction, all of those people who attended Lower Park Road, Hastings, immediately following the murder of Billie-Jo Jenkins.

"As far as we are concerned, no individual is unaccounted for.

"In the absence of significant, new and compelling evidence, there are therefore no new viable lines of enquiry for us to progress at this time."

Sion, however, is not satisfied with that answer and plans to take matters further.

"It's not over yet, not by a long shot," he told the Observer in an exclusive interview on Wednesday.

"In their letter to me they say the person seen by me must have been one of those people who had legitimate access at the time following my telephone call to the emergency services.

"I am writing to ask them who the person is or might be.

"If the police are saying it's not a police officer, I need to know is it somebody from the ambulance service or somebody else from Lower Park Road.

"Who is this 'legitimate' person?"

He went on: "When I wrote my original witness statement directly after Billie's murder in February 1997 I signed that statement with the police and they have never contradicted what I said.

"They have never said that this person does not exist.

"It's no good the police telling me now after 11 years that everyone is accounted for.

"I am determined to find the person who murdered Billie."

Sion was originally jailed for the murder, but acquitted in 2006 after juries in two re-trials failed to reach a verdict.

He is currently awaiting the outcome of a compensation claim relating to his imprisonment.