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Are the tectonic plates of British politics moving with the formation of a new "centrist" group in Parliament - or are we experiencing a minor tremor?

It's too soon to say - but that won't stop some MPs and commentators declaiming as though they were expert political seismologists.

So let's stand back and examine the landscape.

A split is quite an easy thing to understand.

But currently there are fissures that run all the way through British politics and which makes the future look far from stable.

The addition of three Conservative MPs to the ranks of eight Labour defectors could, on the surface, look like the breaking of the British political mould.

But let's examine the multiple fissures more closely.

Timing is everything

Momentum - the influential group of Labour left wingers - has denounced the nascent political grouping as neo-liberal Blairites and Tories.

Yet not many "Blairites" or centrists have, as yet, signed up to this project.

One reason is Brexit - the primary reason for the breakaway, according to defecting former Labour MP Chris Leslie.

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Senior members of the People's Vote campaign for another EU referendum - who most people would regard as ardent supporters of the last Labour government - pleaded for months with Chuka Umunna not to set up a breakaway group before Brexit had been settled one way or another.

They did not want People's Vote to be seen as a de facto new party because they felt this might breed suspicion among left-wing Labour Party members who are far more pro-EU than their party leader.

Leading lights in the People's Vote campaign wanted to detach these Labour members from Mr Corbyn, so that they could back a new referendum without feeling disloyal.

But the defectors have gone over the top now - they have formed what looks like the beginning of a new party.

They might have been able to take more "centrists" with them had they waited.

Splits in the centre

But timing and tactics aren't the only divisions amongst the so-called "centrists" in Parliament.

This is an over simplification but essentially the centrists split in to two groups.

First, the defectors, along with those who are all but ready to defect or for whom it wouldn't take much to push over the brink that they have precariously occupied.

And secondly, those who will "stay and fight".

The success of the breakaway will - in part - depend on how many will move from group two to group one.

Labour and Tories together

Essentially, the current fissure is based on those for whom "stop Corbyn" is their overriding objective - and those whose fundamental deep seated raison d'etre is to Stop the Tories.

And the sight today of former Labour MP Ann Coffey chatting away, in apparently chummy terms, to former Conservative Sarah Wollaston on the same Parliamentary bench will make it more difficult, not less, for the Independents to attract further Labour support.

Having said that, I still expect to see a few more defectors - the Labour leadership expect a dozen in total to go.

Incidentally, it may also limit the appeal to those Labour voters long uncomfortable with Mr Corbyn but who have felt they had nowhere else to go.

Former Scottish Labour leader Jim Murphy told me the story of when he canvassed a voter very disillusioned with his party on a number of issues including immigration.

"I was thinking of backing the BNP then I thought 'naw, that would just let the Tories in'", said the voter.

In some parts of the country a group which takes in former Labour and Conservative MPs could be seen as refreshing - but in others it will be toxic.

Red Tories?

The Conservative defections have also allowed abrasive left wingers to say they were right all along to paint Chuka Umunna and chums as "red Tories".

There is a debate in Labour leadership circles about whether to call another confidence vote in the government, in the hope that the new group will vote with Theresa May and be depicted not so much as red Tories but actual Tories.

One prominent "centrist" Labour MP told me privately he was pleased some of his colleagues had gone as he would no longer himself be "tarred" with the accusation that he would leave - or that his loyalty wasn't first and foremost to the party.

The left's dilemma

But there are also divisions within the Left on how to handle this.

Is it better to be conciliatory and try to address not just the defections but the causes of them?

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This is the approach favoured not only by Dave Prentis - the general secretary of Britain's largest union Unison - but privately by some much closer to Mr Corbyn.

But others want to "clean out the stables" and step up the de-selection of the Corbyn critics who remain in the party.

Some close to the leadership do not want this to happen - but admit that controlling some of the activists who have joined the party in recent times isn't an easy task.

One left-wing insider told me that they had been genuinely shocked at some of the examples of anti-Semitism in the party but trying to convince some rank and file members that the allegations and investigations were not part of an anti-Corbyn plot was a forlorn task.

So the number of future defectors may depend on how disciplined and measured the reaction is from the Labour leadership's supporters in local parties. Some MPs could yet feel "forced out".

Electoral gamble

Where the Left is united is in calling for the defectors to stand down as MPs and fight by-elections.

Many of those MPs have large majorities and, don't forget, many of them would - as we revealed at the last election - have barely mentioned the Labour leader in their 2017 campaign literature and instead punted the message that Theresa May needed reining in.

And both Shirley Williams and Roy Jenkins scored spectacular (but short lived) by-election victories in the early days of the Social Democratic Party, which broke away from Labour in the early 1980s.

So the Left may have to be careful what they wish for.

A couple of members of the new Independent Group are said to be considering putting themselves in front of the electorate.

But this is high risk.

So far, we know more about what this new group is against than for.

( Indeed although they are currently all pro EU some of their potential future members are much more Eurosceptic.)

And possibly for former Labour MPs the biggest risk in a by-election would not be defeat by their old party but so dividing the centre-left vote that a Conservative wins.

That might do more to herd some potential defectors back in to their Labour fold.

Challenge for the existing parties

Any anti-Brexit former Tories would face a brutal campaign which would seek to rally pro-Brexit voters by portraying the defectors as part of a political establishment which would betray the verdict of the people.

But perhaps the way the new Independent Group might change the political dynamic is this - their mere existence tells the leadership of the traditional parties that if they don't listen to the concerns of their parliamentarians they - and some their voters - really do have somewhere else to go.

So they present a challenge to those at the top of the existing parties.

How - and if - the leaderships of these parties change could determine whether the defections eventually register on the political Richter scale.