If it's a small gap you could fill it with black silicone.
Matter of taste, mug flaps. I put them on my first car because I was young, daft and just wanted to do affordable things to my car. Not bothered with them since although they do dramatically cut down on damage from stones. My wife's car came with them, it was a demonstrator and had a few extras.
I used to have them on my cars up until 2016. There wasn't an OEM set of mud flaps for car I bought (VW Polo GTI) in 2016 and I’m not a fan of aftermarket universal mud flaps (poor fit on some cars) so cars I’ve owned for the last nine years have been without mud flaps. I have to say in all honesty those cars without mud flaps haven’t suffered any worse from stone chips or dirt accumulation than previous cars owned with mud flaps fitted, although I appreciate the types of roads a car is driven on will be a factor in how vulnerable a car will be to collecting stone chips and mud (I rarely drive on muddy rural roads).
Looking at the OEM mud flaps for the Leon, they do seem quite short - especially the rears for the ST (see picture below) so I wonder whether they’re actually very effective at providing much protection, other than to a quite small area immediately behind where the mud flaps are fitted. Being quite short, I also wonder whether they provide much benefit from road spray for following drivers
