...I asked one about whether I could pay contactless. Avoiding eye-contact, he replied "get the bus at stop E1". I made a second effort - to which he said, I could buy a ticket at the machine if I wanted to and then wandered off. Obviously incapable or unwilling to understand my question....
I suspect he was either hinting to you there was no requirement to pay for the journey, or was so used to the idea that the journey would be free that he couldn't understand why anyone would be insistent on paying for it.
... it came to an argument I could show (albeit later) from the TfL record that I had touched in....
There wouldn't be an argument as such; if an RPI read your contactless card and you were not touched in, the RPI wouldn't even know. It would just result in a maximum fare, which a phone call could probably resolve, in the circumstances described.
Bus was 15mins late. Until it turned up, none of the "can I help" guys knew what was going on. All chaotic. People piled on and a bloke with the driver announced "fast to Gatwick". A few people complained "what about Purley" (where I suppose it was meant to stop). Then there was a rumpus.. apparently some wanted Hooley too. They were told to get off now or "enjoy Gatwick - don't talk to me about Hooley or wherever". We departed. Nobody bothered with checking tickets and telling pax that they should have them.
Sometimes rail replacement buses are effectively free, but even where that is the case, it's unlikely to be advertised as such, for obvious reasons.
For example, LU do
not require passengers to touch in on rail replacement buses, but passengers simply have to demonstrate a willingness to pay by holding a contactless/Oyster card with sufficient credit or a Travelcard (or, obviously, a valid paper ticket).
This contrasts with SWT who went out of their way to install standalone readers by bus stops, at certain locations.
At Gatwick I went to the barrier. Told the bloke I needed to "touch out". Took a bit of explaining, but then he laughed, used his card to let me in, so I could then "touch out". I asked him if it was the first time he saw anyone do that - he said it was. "Man you're probably the first person to pay for that journey".
It's correctly gone through as £3.
Your determination to pay is admirable, but even you would find it impossible to pay for some journeys on Northern!