Mytrip.com complaint: Their Technical error cost me £460

I booked fights and due to Covid changed the dates.
When I called again to put a date, they calculated fare difference and admin fee.
They sent me a link to pay £60. I paid it and was told that I will receive tickets within 24 hours.
I waited for it but nothing came up.
After 24 hours I called and they couldn’t find anything on the system.
They said there was a technical error and the date change didn’t go through. If o want to fly out on that date I have to pay further £460.
I explained again and again but was told you can either cancel the ticket or pay the money.
I asked them to calculate my refund and was told there is no refund amount.
Basically I was bullied and blackmailed to pay further £460 for their mistake as I had no choice.
I want a refund of £460 plus compensation for giving me so much mental stress and tension.
Overall their service is rubbish.