Conversely, I can understand someone sitting in (my) reserved seat if there aren't (m)any seats available, if the train is (near) empty, why anyone would *choose* to sit an an obviously-reserved seat is beyond me - hence I *always* ask them to relocate in these circumstances. Sorry, but why sit there when there are plenty of *unreserved* seats is beyond me....and serves you right to having to move, frankly.
No reservation flags on the seat or anything.
TM▸ agreed with me and told the suit to go away and sit somewhere else. Suit was not happy...

What I was getting at is if a seat is
NOT marked as reserved and mid journey someone tries to eject you from the seat as per my case above.
If I see a seat is reserved I'm not going to sit in it! If I did for whatever reason I would expect to have to move and would do so.
I may have a good eye for pedancy and detail but I'm not a mind reader for knowing which seats are reserved but not marked as such...

Maybe I'm being naive. But if the seat is not marked as reserved and someone asks you to move mid journey when there are plenty of available seats (I'm suggesting like 90% of the carriage for First Class) that to me just seems far too precious and defies common sense!