Lime plaster filler

Hi all,

i found a small piece of my lime plastered wall (2 years old) had fallen off onto the floor. The piece seems to have come away right next to the window frame. These old houses flex a bit with the seasons but can anyone recommend what filler to use?

Thanks

 

Thanks Astramax. Forgive my ignorance... Is it different to bog standard filler, does it allow any breathability?

 

Super. Yes, I used clay paint so it's breathable.