The body of 42-year-old Kevin Gough was believed to have been there several weeks, The Daily Telegraph reported Friday.
Employees at an attorney's office said they discovered a gap in a wall in an unused part of the building about a month ago that they decided was the result of an attempted break-in but did not make a report to police.
Officers were called to the building Wednesday after workers reported a sickening smell.
Recovery of the body was complicated by the historic nature of the building and specialists were called in to disassemble the chimney brick by brick, officials said.
Gough had been convicted and served two years in prison for a string of business burglaries.