Does anyone know what this trim/section is called?

[Hightowermark]

Replacing a damaged UPVC window and it's an old 1920's house which has a curved lintel (like the photo) but what is the curved section called, or is it fabricated on the window itself?

thanks

 

[crank39]

It's cut on site to an arch from 5m lengths of pvc flat board or pvc trim, any building plastics supplier like eurocell can supply it