So: UKIP is now filling the (small) void left by the disappearance of the BNP. The ERG want the Conservatives to fill the void left by UKIP. And now the Independents have noticed that there is a bit of a void left by the shrinking of the Lib Dem party.
Regarding fighting a by-election: I think they have precedent on their side. I haven't got any statistics, but I get the impression that in the past only a minority of MPs who have defected ever resigned to fight a by-election. And there is the legal point that we vote for the MP, not the party. Having said that, there is certainly a strong moral case that they should trigger by-elections; although personally I would prefer to just have a general election.