To start off with then, here’s a photo of the ‘gap’ behind the bathroom sink, where any previous sealant has crumbed and broken away. At a guess, I might question whether the correct grade of sealant or caulk was used. In front of the splashback, it is subject to a high volume of water at times. It seems to be fine in other places.
If you were to look more carefully from the right angle though, you’d see that the sink unit runs at a different level to the window sill behind. This suggests that’s something’s out, if the base of the unit hasn’t deteriorated… Maybe I can level it, once I get inside and underneath.