Green’s Dictionary of Slang

phr. indicating scepticsm and rejection of another speaker’s statement; also giving an unstated 'yes' to a given question.

1962		J. Wynnum Tar Dust 30: ‘Nothing much you could do about that now’ ‘Not many, Benny!’.
1999		J.J. Connolly Know Your Enemy 151: I’m starting to lose patience with him,’ I says. ‘Lose patience, not many benny. He’ll end up going completely mad with Roy’.
2005		R. Blonde T-shirt Theory [ebook] ‘Morning Dad. Kettles just boiled wanna cuppa?’ Gary offered. ‘Not many Benny’.
2023		P. Bullinger Reeling Back the Years [ebook] The phrase was ‘not many Benny’ when asked a question to which the answer would be a ‘yes’.