Gap between liner and tiles

UKjames

Silver Supporter
May 1, 2020
92
United Kingdom
My liner seems to be sagging away from the tiles. It’s worse in some places than others - perhaps an inch in the worst place.

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What material is best to use to fill/finish?
 

UKjames

Silver Supporter
May 1, 2020
92
United Kingdom
any brands / products recommended?

my initial thoughts were a bathroom silicone as I guess needs to be able to flex a bit but that doesn’t really feel like the right stuff for the job.