As the initiator of this thread, I appreciate the many helpful comments posted and being given the chance to meet requests for further information.
Was I being rude or aggressive to the Lumo inspectors? Certainly not. I'm a retired teacher and probably look like one. I probably displayed exasperation at the repeated demands for further payment, when I could show I was following advice which seemed to say I didn't need to make an additional payment. Exasperated, but never rude or aggressive.
What happened to the other two passengers who got off at the unscheduled Doncaster stop with me? They were gentlemen of a similar age; I spoke to them, enough to ascertain they were in similar circumstances to myself and hadn't met the demands for an additional fare. They told me some much younger men on the Lumo train had been pressured into making extra payments of "around £40" each. One of the lines the Lumo inspector had taken with me was "Everyone has paid except you". When the following LNER train arrived, we boarded separately and I never saw them again.
My ticket details? There were 3 tickets with the same coach/seat reservation on each of them (Newcastle-Durham; Durham-Doncaster; Doncaster-London Kings Cross). All were shown as "Advance Single" "valid on 01-FBY-23" "Adult Standard Class with Senior Railcard" (I had my Railcard with me). The first ticket specified "On London North Eastern Railway Services 10.27"; the other two tickets showed "On specified LNER trains & connections" with specific subsequent departure times [see attachment].
However, the issue affecting me was the emailed advice which TrainSplit gave. This said that as my specified train had been cancelled:
"You do not need to purchase additional tickets to complete your journey.
Take the next available service which matches the route and operator restrictions on your ticket(s)."
I naively took "operator restrictions" to mean restrictions imposed by the operator. It did not occur to me that it might mean the passenger was restricted to the same operator as the cancelled train (an ambiguity which TrainSplit have acknowledged in a helpful Customer Care reply to me). On 1st Feb, I'd seen online there was a 10.55 from Newcastle after my cancelled train; so I caught it, in line with the advice I'd been given. It happened to be a Lumo train (indeed, I wasn't even sure there were any LNER trains running from Newcastle on the day, because of the strike).
Being stopped by British Transport Police and the Lumo inspector at Kings Cross, after travelling from Doncaster on a LNER train is incredible? Yes it is, as is the whole disturbing saga. But worryingly, it's true.
Thank you for the collective advice of the Forum.
View attachment 128450