"One session I go to is being wrecked by one person who takes turn after turn - sometimes singing 3 or 4 things in a row. He then lets someone else have a turn but jumps in again straight after. No-one wants to upset him by telling him to shut up and let so-and-so have a turn because he's a nice old chap and we're sure he means no harm but it can be difficult not to look bored (or exasperated) when he hogs yet again."

I have met a number of younger versions of this guy in sessions/singarounds in various locations in Yorkshire - trust me when I say they won't be any different when they're long in the tooth and in the category of "nice old chap". Have a quiet word with him - ask him if he wants to go on an extended Saga holiday.