if this independent group do start a party are they actually as big a threat to tories as they are labour?
No not really, at least in the early stages.
For every 9 voters they tear away from Labour in their seats there'll be one Tory at best. And by and large the seats they're in are in Labour strongholds anyway, so that vote loss for the Tories isn't as big a factor.
Long term, the danger to the Tories is that they bleed votes to the centre ground if they continue to veer right, and the natural inclination of this Independent Group is to coalition with Labour. But you're talking a good five years or so and the likelihood of them existing at that point is negligible.