Hey there
We just got granite worktops in a few weeks ago and when I saw your post I began to panic!
When we got our worktop the guys installing it put some sort of stuff on it to seal it and keep it shiny. Sorry I can't remember the name, but essentially it's a liquid that you rub on, leave for a few minutes and then buff to a shine. Whoever installed your worktop should know, or any other granite worktop supplier.
It needs a bit of elbow grease to buff so be warned!!!