SINGING star Herman of the Hermits, who first became known to millions as Len Fairclough's son in "Coronation Street," yesterday signed a contract that makes him Britain's youngest millionaire.     Herman - real name Peter Noone - did it by clinching a disc and film deal in London with America's M.G.M. studios to make at least three films and several records over the next three years.     Last night Britain's latest mop-haired member of the million-dollar league rang his parents in Huyton, Liverpool, to break the news.     Said 18-year-old Herman: "They were delighted but I still can't believe it. It's a fantastic opportunity, and we're looking forward to it tremendously."     As well as the guaranteed million dollars, Herman will also get a percentage from the takings. The final value of the contract could be £1,650,000. Very proud     Said his father, 42-year-old accountant Mr. Denis Noone last night: "There is no doubt at all that Peter is now a millionaire."     He has been close to this for some time, but this contract makes it absolutely certain.     "We are very proud of him, at having done so well in such a short time."     The first film "Hold On," scheduled for release in September, will be followed by two		more in which Herman and the Hermits star in singing and acting roles.     The second film will probably be based on their hit-song "Mrs. Brown You've Got A Lovely Daughter."     Final details will be drawn up during the group's six-week American tour, starting next Monday.     The film contract comes just six months after Herman turned down a similar offer from M.G.M. to star in four films.     Reason given at the time was that the guarantee to pay up by the film company was not firm enough.     After the contract signing yesterday, Herman and his group held a celebration party in London. Champagne     But although he handed out champagne, he asked only for bitter lemon.     And when it came to wanting a meal, he said: "I'm popping across to a transport cafe run by a Greek. Chop and chips. Lovely. Quicker than the Savoy, too.     How did success start for Herman?     He thought it was in the school hall at St. Bede's, in Manchester. They were performing "The Mikado," and he was 11.     "I was one of the three little girls from school," he said.     "It was Yum Yum, I think. No, maybe not. But we got a rave write-up in the school magazine."