There are plenty of variations and they differ even between families, let alone regions.
I knew all four of my grandparents and three of my great-grandparents and I called them:
- Jo and Des (my mum's parents) - those were their first names, also used by my mum and her two sisters, who didn't use "mum/dad"
- Gran and Pete (my dad's parents) - I have absolutely no idea why it wasn't "Gran and Granddad" or "Phyllis/Phyl and Pete" (their actual names).
- Nan and Pop (my mum's maternal grandparents) - I think I just copied what my mum called them.
- Nana Trumble - (my mum's paternal grandmother) - again, I just copied what my mum called her.
So, as you can see, within my family, the terms "nana", "nan" and "gran" were used, as well as actual names.