Hi,replacing a turbo on a blt vrs,quite a lot of the impeller has broke off and made its way around.will it be best to remove all pipes and clean them and flush cooler or is it best to replace the cooler?

I would clean and flush all the pipes and get a new intercooler if it was me

Just check the intercooler has caught everything.  I'd be tempted to remove and clean the EGR & inlet manifold, just as a precaution.

you can send intercoolers away to be properly cleaned .... and yes +1 on removing inlet too^^  but really not much should pass through the intercooler