A follow-up documentary to the ITV1 programme which first revealed allegations against Savile airs tonight on ITV1.
Former police detective and child protection expert Mr Williams-Thomas was behind ITV's Exposure documentary about Savile, which aired in October.
In tonight's programme, he claims ITV's investigators have new evidence to show that Savile was a life-long sexual predator.
He said: "In the first programme, we focused on a fairly short time scale.
"Now we're able to show that Savile's offending behaviours spanned four decades and many hundreds of victims.
"This isn't just someone who offended only against 13-, 14- and 15-year-olds. It's someone who offended against 10-year-olds."
Police have confirmed 500 people have come forward as victims of Jimmy Savile.
The investigation into the Jimmy Savile abuse scandal has cost around £2 million so far according to the Metropolitan Police Commissioner.
Jimmy Savile thought he was "untouchable" and would regularly walk in on Broadmoor patients while they were in the bath, it's been claimed.