How much money should the sponsor show to be granted visa to the UK? [duplicate]

I'm hoping to apply for the UK visa, but have been holding back because I'm worried I'll get refused (I'm Sri Lankan, refusal rate is high for us!).

I'm a student doing my Master's course in Russia. Almost each summer when I was doing my bachelor's degree (also in Russia) I traveled to many East-Asian countries like China, Japan, Singapore and Thailand.

This year I'd like to visit the UK (in winter because I got an internship during summer).

Since I'm a student, I only have about £450 in my own bank account. My mother will be sponsoring me. She has a fixed account with £5,000 and is hoping to spend £1800 out of it for my trip.

She'll be left with £3200. Her pension plus my father's salary is £300 per month and my father's fixed account deposit is £5000.

(My married sister who earns a £1000 per month takes care of utility bills in the house, because her husband earns more than enough).

I want to know whether they'll think this amount is okay for my mom to get by on after paying for my trip? I get free tickets each year through my sister (she works at an airline).

If they do, then will they accept fixed deposits?