The NHBC are unfortunately correct as this looks like an aesthetic defect rather than structural although it is clear from their reply, they don't know the difference between a "sill" and a "lintel!"
From your photo, it looks like the manufactured stonework head is bedded on a steel lintel which is what provides the structural support. Quite why the decorative stone head has cracked so badly is a mystery. In all likelihood it was already damaged when fitted and the crack has just opened up in the weather. These stone features are on a long delivery and it is "easier" for the site to just some stone powder into the crack so buyers don't notice it until it is too late!
The NHBC should at least inspect and investigate and make sure this is not the first sign of something more serious like subsidence.
As for what to do now you can only write to Persimmon CEO Jeff Fairburn and tell him you are not happy with this.
I very much doubt Persimmon will do anything though as it is over the two years.
Your best option would be to write to the manufacturer, and ask for someone to carry out an epoxy resin repair which I am afraid, you will have to pay for.
With home ownership comes maintenance obligation, even with a new home.