How does one go about harvesting this history
It's as simple as writing something down whenever you hear an 'occasional thought'! Maybe write something in a notebook or something so that you can pass it on in the family should someone show an interest. It's these small moments that can reveal something interesting that pushes you off in another direction. Record them while you still can.
Earlier in the thread, Al, you said this:
What he talks about is not the operational flying as such [...] but instead the esoteric craic, the gossip and the banter - the colour on the tapestry that we are going to lose because it wasn't and couldn't be properly captured and transcribed.
It's these stories that most fascinate me. You mght not be able to completely capture
What It Was Like, but you can give it a red-hot go!
Eventually you might like to do things like researching service records or photos. There are a few websites and things around that could be of use; flick me a PM and I'll fill you in if you're interested.
Declaring my interest: I'm currently researching a great uncle who was a navigator with 467 Sqn. He was shot down and killed over Lille in May 1944 so obviously I never had the opportunity to talk to him about his experiences. So I've had to resort to letters and diaries and logbooks from the period (and reading the thoughts of veterans like Cliff and Reg on 'that other thread').
Even if you never share it with anyone outside of your family, at least it will be recorded somewhere for future generations. That way, men like John and Donald will not be forgotten.