Window to cladding gap

[bsr]

Hi, as you can see there is a gap between my Windows and the surrounding PVC cladding which is moving too much for the sealant to handle. Any suggestions how to seal it please? Water is getting in and tracking down to the ceiling below. Is backer rod an option?

Also, any idea what could have pulled up the flashing under the cill in the first picture?

This is a PVC clad timber dormer which might explain the amount of movement.

Thanks