Filming took place during Easter, it had been a drag waiting until now to come clean, "especially being a school teacher".
Since then everyone had been studying his body language and face, he said.
Mr Barnes spent four months studying from two reference books which the the questions were taken from.
Study sessions were broken into small chunks, sometimes he slogged away for four-and-a-half hours, broken into 90 minute blocks, and studied for 90 minutes each day after work, using cards and timelines.
"It was quite fun, it didn't feel like study, it was great sucking up all that knowledge."
While not putting himself under any pressure during the show he said his main fear was "bombing out".
His friends and family were in the audience, but getting a main line of sight towards his wife was essential, he said.
Surprising himself, he said his best round was New Zealand sport, remembering things from his childhood, answers "just came flying out".
The most nerve-wracking on it all? Waiting for his turn in the hotseat.
"But once you're in the chair it's fine because everything turns black and it's just Peter Williams and the spotlight."
His secret was to psyche himself up to a point where nothing else mattered, just 100 per cent focussed on the questions.
"I really liked it by the end, it's quite addictive. Once I knew I wasn't going to bomb I thought, this is so cool."
Remembering the show from his childhood he was still grasping the idea that he, like those he had idolised, was now a Mastermind champion too.
Inheriting a love of general knowledge from his mother, he said he had always enjoyed trivia, general knowledge and pub quizzes.
Now a proud owner of the Mastermind chair and sculpture, the champion said when he saw the show advertise last November it was something he "just had to do".
Teaching, listening to music, watching football, going running and reading, his latest books being Game of Thrones, the champion said he would definitely defend his title if he had the time.
"If this is something you are passionate about and you are prepared to put in the time you've got nothing to lose. If you get onto TV then you're a winner."
