Vistara — Refund of cancelled flight

My PNR was SASNLU
I was suppose to fly with Air Vistara on 23rd December 2020 from Ahmedabad to London Heathrow. Due to Government ban, all flights were cancelled from 22nd December 2020 night, I have called air vistara so many time since 21st December 2020 regarding this issue but they told me that I will not get any refund. When I counter that saying government told airline to do so than rudely guy on the other side told me that get this from government in writing than talk to us.
They also told me that because flight were cancelled now I have to pay £125.00 + fare difference which was at that time £1, 300.00 on economy one way from AMD to LHR. this is rip off. rather then helping people out they took advantage of situation and keep taking all money out of customer's pocket.
I checked my vistara account today and all my booking history got vanished. it looks like I have never traveled or booked with them before.
I have complained to air vistara in writing on an email, after 5 days and 2 reminders no reply from them where their customer standard saying that they will reply in 24-48 hours.
I have no clue where to go for my money????? As I am back to UK and they do not have any local UK number for customer care, I have to pay fortune to contact them. Please get me out of this thug air line with 0 customer service.