I have just had a new steel garage door fitted, and it has left a
small (gap 1-2cm high) between the top of the door frame and the
brickwork of the garage itself.

My (no doubt stupid) question is, what should I use to fill the gap?
Cement or a mixture of cement/sand etc?

Please let me know what you think, as its a simple job, but I really
don't know what to use.