For those not in the know, in the 1970s the British Labour Party moved further and further to the left, to the point where a "Gang of Four", four senior moderate Labour politicians split off from Labour to create the centrist Social Democratic Party, which then proceeded to form an electoral alliance and eventually merge with the already existing, and on life support, centrist Liberal Party to form the modern Liberal Democrats, who up until the rise of the Scottish National Party in 2015 were the third biggest political party in the UK.
What if Tony Blair's New Labour ends up shifting Labour so much to the Centre (not being helped by his pro-Iraq War attitude and soft austerity policies) that four very Left-wing MPs, Jeremy Corbyn, John McDonnell, Dianne Abbot and, most prominently, Dennis Skinner decide to become the new "Gang of Four" and split off from Labour to create a new, genuinely Left-wing political party?
What are the likeliest scenarios to follow such a move? Does the new Left-wing party merge with the Greens to create a stronger and more viable party that does just as well if not better than the LibDems (and possibly siphon some of the Left-wing vote that Blair absolutely ignored)? Do they peter out and Labour remains perpetually in the Centre without Corbyn to pull it back to the Left? Do they end up forcing Blair to turn Labour back towards the left in an attempt to stop the exodus of centre-left and left-wing voters to the new party? Or does something else happen instead?