The discovery came after an exhaustive investigation involving interviews with TMB board members, staff and volunteers, as well as Sir Norman's family members and regular visitors to his home.
Speaking earlier today, Sir Norman said he was pleased that the matter was now resolved. "I can't pretend it was pleasant having to interview my wife and mother-in-law, but they were given the right to be accompanied by a friend and, in the end, were exonerated," he said, adding: "It's all cleared up now and we will be hiring a new firm of cleaners."
The investigation followed the leak of private correspondence to celebrated political pundit Dan Hodges, self-styled 'Poverty Tsar' Frank Field MP and, most recently, to Johnny Mercer the MP for Plymouth Moor View.
The latest leak had been particularly embarrassing as it accidentally revealed the TMB's planned post-Brexit marketing campaign for tripe. 'Tripe - it's tolerably tasty' was to have featured on posters at bus stops and at selected motorway service stations in the south of England, but Sir Norman said that the element of surprise had now been lost.
He took the opportunity to apologise to Mr Mercer for any distress caused by the leaking of the letter. "I'm naturally sorry if this has caused him any distress, but I can confirm that the offer is still very much on the table," he said.