It does happen on the 6.41 (usually at the front coach of the 4) but happens more often on the 7.16 to victoria and very often on the 7.33 to London Bridge. It even happened on the 16.53 coming home last night with some girl wanting to save a seat for her boyfriend who will 'only be 5 mins'. The guy opposite me told her to go to another carriage (the 3 + 2 across ones) where there would be plenty of space. I kept my head down (dont do confrontation on trains )
The 6.41 is the train I usually get but I'm hardly ever in the front coach so I miss out on all the fun. I've never seen it at all, I'd be ready to bite someone's head off if they tried it with me.
I do sometimes wish that I could get a seat with three bridge players so we could get through a quick rubber or two during the course of a week - that would pass the time nicely.