gap appearing above windscreen

Hi,
I am surprised to see a gap a few mm wide appearing in the joint directly above the windscreen. Where the fibreglass high top is joined on.
I propose to reseal it with black Sikaflex only.

anyone any experience of this?  

2006 Sussex/Symbol well cared for 53,000 miles

possibly completely unrelated, I am getting water stains in the roof seemingly related to the large heiki roof light, I will try and reseal this.  I did not expect these two jobs on a young van (just ten years old)....yes I know this old to some..

Water water every where and not a drop to drink...

thanks for any thoughts 
Graham