remove C pillar section due to boot water leak

[Robo70]

I have a leak in the boot which ends up collecting water in the spare tyre well. After much investigation it appears to be coming through the base of the c pillar where that detached part is. Does anyone know how to remove that part so I can silicon it up and stop water leaking into the boot?
:cry:

 

[markovr]

Are you sure its not coming in through the fixed back window as the windscreen sealer that seals it in time can leak and let water into the boot.Thats a matter of unclipping the plastic trim inside and undoing the plastic nuts holding it to the opening resealing and reciprocating.

 

[Robo70]

I think you might be right. Thanks for the advice.