I have been reading this thread with interest.
Being a bit of a cynic it crossed my mind that the “sons” might be making the whole story up, or perhaps their father spun them a web of fibs which they are perpetuating, or the three of them may have been in cahoots? It is a bit suspicious that no trace of any of the main player’s births can be found, and now one of them is (conveniently?) too ill to be contacted.
It brings to mind a story written by someone my father in law had a tenuous connection to during WW2. He wrote of his exploits in the Special Branch and as a bodyguard to famous people. Most (or all) of the events were true, but the manner of writing was all very “boy’s own” and enhanced for dramatic effect.
Like others have already suggested, a name change could explain the lack of records about this man, however the surname Munnings is an odd choice if someone was trying to cover their tracks, unless there is some family connection to the surname. The surname seems concentrated around the SE - Suffolk area.
Added: if this seems too harsh an accusation, please ask me, or a moderator, to delete my post. I would not want to wrongly accuse someone especially on a public forum.
I hope you get to the bottom of it all smudgepants.