HERMAN, who went "absent without leave" in Ibiza, Spain, causing him to miss TV and Radio 1 recordings, is to enter the field of cabaret for the first time this autumn.     On return from an American tour which opens next week, Herman's Hermits do a Northern cabaret date as an experiment. Manager Harvey Lisberg told Disc: "At the moment, of course, they are far from being a cabaret act, but we have had so many inquiries for the group that we will do a booking as an experiment."     Herman returned to London last week from his Ibizas holiday three days late because, he claimed, he failed to receive messages recalling him for promotional dates for his new single "Sunshine Girl."     He's now apologised to all the people concerned and offered to do a free appearance at an American Air Force base he let down," said Lisberg.     Group flies to the States on Sunday and will be away until August 25. Part of the tour will be taken up promoting their new film, "Mrs. Brown You've Got A Lovely Daughter," on current American release. Film is likely to be shown in Britain from September and one of the songs featured - "The Most Beautiful Thing In My Life" - is to be released as the group's next single.