What happened to Naruto's grandparents? Was Kushina the last of the Uzumaki clan and if not why did none of her family take Naruto on as an orphan? Who was the Namikaze's and why have we never heard of another one?

Were both Kushina and Minato orphans themselves? What is the story behind Naruto's extended family?
The Uzumaki clan and it's village was destroyed some time after Kushina came to Konoha. Survivors went into hiding and and spread throughout the world.
It is unlikely that direct relatives of Kushina survived.
Regarding Minatos parents we just don't know anything about them.
But considering that the third shinobi world war took place at that time where even children barely out of the academy were send to fight it might be a reasonable assumption that his parents died in it.  

Sources:
Minato
Kushina
Uzumaki clan
3rd Shinobi World War