A builder put in a new window-sill in our kitchen
a year or so ago, and I just noticed that
there is a large gap - up to 1 inch at one end -
underneath the sill, and there is a strong draught
through this space on windy days.

I'm wondering what is the best way to fill this gap?

I should say in explanation that the sill was put in
when our sash-windows were restored.
We didn't actually ask for the sill to be put in;
I presume the builder did it because he thought
the existing setup did not do justice to his window.

Also the space is probably there to level the sill.