It was longer than 40 years. Mike joined TTTV as a newsreader/reporter/continuity anno in 1962, which makes it 43 years.
TTTV made a point of calling Mike the UK's longest continuously-serving local news presenter, so surely there must be some truth in that.
Of course, FD will take that role now since MN has retired.
As for the Dine(n)age spelling, I think all you need to do is look at the quality of the entries for "Fred Dinenage" on Google vs "Fred Dineage". The former are largely official pages (meridiantv.com, imdb, wikipedia etc etc) whereas for Dineage, the pages are mostly for forum pages and the like, and Google itself asks if you meant "Fred Dinenage". Not conclusive but I think it's a safe bet.
Always amusing when Nick Hervey gets something wrong