New British citizenship application by double descent

[taurusUK]

I'm aware this gets asked a lot but I have realised things differ so much with each situation I thought I had best ask.

I was born in South Africa but am a british citizen by marriage 16 years ago. My mother would like to come to the UK and has been told she can do this by "pre-1983 double descent". She was born in Kenya but is registered a south African citizen. Her mother was also born in Kenya, but also never became a citizen and was South African up until her death.
My mother's mother was born in England in 1901 and therefore british by birth.

So it seems I can use form UKM for this. I am guessing my mother will need her birth certificate, marriage certificate, her mother's birth certificate and her maternal grandmother's birth certificate?
Correct me if I am wrong...

Would she need my father's certificate of death as she is now widowed? Is it passports or certificates needed or both? What if the birth certificates aren't available (I have checked and her maternal grandmother's replacement one is available to order from GRO if needed).

Also it seems on the website that the fee is just £80? Is this correct - surely not!

Thanks in anticipation of any advice.

[taurusUK]

Can anyone help please?