Hi Gaccha, thanks for the reply and yes, the table says this: If your parents were born on or after 1 January 1983 - evidence of your grandparents’ claim to British nationality by providing their birth certificates and, in the case of grandfathers, the marriage certificate to your grandmother.
However I cannot provide my child's British grandfather's birth certificate as he left me when I was a baby and I don't know any of his family's details so I can't order his birth certificate, he also changed his name when he was a teen and I don't know the name he was born with.
Anyway I ended up ringing HMPO in the end and they confirmed just need to write a separate letter with as much info as possible of my parents, full name, date of marriage, divorce, name changes etc. And they should be able to verify and if they can't, they'll contact me to send in my mother's birth certificate etc. But they said it rarely ever happens unless there is a big red flag and in my case should be fine.