New evidence obtained by the Guardian suggests that the senior Conservative figure at the centre of sex abuse allegations broadcast last week by BBC2's Newsnight has been a victim of mistaken identity.

A local councillor who was himself a victim of abuse at Bryn Estyn, the Wrexham care home at the centre of the allegations, told the Guardian on Thursday he did not believe Lord (Alistair) McAlpine was involved in the scandal.

McAlpine, who served as a Tory treasurer under Margaret Thatcher, has been widely named on the internet after another former resident of the home, Steve Messham, told Newsnight that he had been taken to a Wrexham hotel and abused by a prominent Thatcher-era Tory figure more than a dozen times.

McAlpine has vehemently denied the allegations pointing out that he lived in the south of England at the time and had only been to Wrexham once in his life. On Thursday Keith Gregory, the Wrexham councillor who has been an eloquent spokesperson for the victims of abuse this week, said he believed a different member of the McAlpine family who lived locally may have been mistaken for Lord McAlpine.

He said a man who children at the home believed to be a member of the McAlpine family would arrive at Bryn Estyn in an expensive car. "He was a right flashy thing," he said.

Lord McAlpine was exonerated by the 1997 Waterhouse inquiry of any involvement in the abuse of children in the north Wales homes but not named because of an order by the retired judge preventing the identification of either victims or alleged abusers. As a result he has been the subject of persistent smears, which resurfaced following the Newsnight allegations about a senior Tory.

But the allegations made by Messham began to unravel after it emerged that he told the Waterhouse inquiry in 1997 that the McAlpine family member he believed to be his abuser was now dead. Lord McAlpine is alive and living in Italy.

The Guardian has identified a number of inconsistencies between the Newsnight allegations and testimony heard by the Waterhouse inquiry.

The mounting concerns over Messham's allegations will be awkward for David Cameron who has already been accused of over-reacting to claims that senior Conservatives were involved in child abuse by ordering two inquiries into an issue which had already been thoroughly investigated by a public inquiry.

Gregory said boys from Bryn Estyn would be taken in "working parties" to the homes of two McAlpine family members in the area – Gerwyn Hall and Marchwiel Hall, both a few miles south-east of Wrexham town centre. "We were like little slaves – we'd do gardening, cleaning the yards, all sorts of things," said Gregory. "We were taken in a van to do work." He said he was not aware of any abuse of children at either of the homes and there is no other credible evidence of such abuse.

Gerwyn Hall, a grand Georgian house hidden by woodland from the public road, was occupied by Jimmie McAlpine, who died in 1991, six years before the Waterhouse inquiry began. Jimmie's wife, Cynthia, still living at the hall, declined to talk when approached by the Guardian.

Jimmie McAlpine chaired the north of England builders Alfred McAlpine Ltd. His Times obituary recorded: "His other main interest was vintage cars, and he amassed what was at one time the biggest private collection in Britain."

The Waterhouse inquiry, when it attempted to investigate the McAlpine allegations, recorded: "According to [Messham's] statement to the police, X [the letter assigned to conceal the identity of the McAlpine family member] had several different motor cars and would wait for him at the bottom of Bryn Estyn Lane."

Waterhouse, a retired high court judge, reported his frustration that Messham would not at the time reveal the alleged forename of the McAlpine he believed had abused him, or who had suggested the name to him. The judge concluded that although Messham was testifying in good faith, and had indeed suffered extensive sexual abuse, his evidence was inconclusive about "any member of the X family". He left open the possibility that Messham might have been wrongly told by a third party that a McAlpine was involved, or have jumped to conclusions.

Messham did not respond on Wednesday or Thursday to requests to comment on the questions raised over his allegation.

Rumours about one or other member of the McAlpine family have accumulated over the years because of a controversial decision by Waterhouse that he would ban publication during the hearings of the names of alleged, but unproven abusers. This attempt at secrecy left the field open for gossip and allegations of a cover-up.

But, crucially, Messham's 1997 evidence to Waterhouse should have ruled out speculation about Lord McAlpine, whose Italian home has been mobbed by reporters for the past week. Reporters covering the inquiry at the time concluded that Messham could not be referring to Lord McAlpine because Messham said his abuser was dead. In another apparent discrepancy, the Times reported this week that their reporter put Lord McAlpine's name to Messham in 1996. "But he said that his abuser was called 'Tom' and had a flat in Wrexham".

The only apparent corroborative evidence about Lord McAlpine has also been undermined. It came from another boy who was not a Bryn Estyn inmate. He described being abused in Wrexham five years later by a wealthy figure with a Harrods charge card. Traced by the Guardian, this victim, who wants to remain anonymous, confirms that his sole knowledge of Lord McAlpine comes from being shown a photograph of him subsequently by a journalist.

The victim told the Waterhouse inquiry, under the name "Witness C" that he was no longer sure he had identified the right man. Waterhouse reported: "C had subsequently indicated … that he could not be 100% sure that his abuser was a member of the X [McAlpine] family, and it is clear that he was referring to a different person."

A BBC spokesman said on Thursday that the Newsnight investigation "set out to explore alleged failures in a child abuse inquiry. An abuse victim had serious allegations to make and deserved to be heard. We broadcast as much information as we had but made clear we did not have enough evidence to name new individuals."