How do I fill this gap in conservatory? Fillers, sealants and adhesives for all types of DIY work. Help, advice and information on all aspects of this subject

[foxlj]

I've got this gap in my conservatory windowsill where the wall of the conservatory seems to have shifted slightly from the main house wall. I can feel quite a big draft so would like to fill it, but have no idea what to use. Any advice?

(Just to note, the conservatory was like this when we moved in 4 years ago but like so many other 'projects' in this house, it appears to have been done "on the cheap")

[welsh brickie]

lift it back into position and pack the underneath, then seal it with white silicone sealant