Opodo cancelled our return flights without notice and will not repay £820 for the of 2 tickets

I bought 2 return flights from Heathrow to Cape Town with Emirates from Opodo, but due to Covid19 our holiday to South Africa was postponed. We were charged £77 when we changed dates , but were unable to use the dates/flights due to lockdown of both countries. Our Credit Card Holder, Santander claimed the £77 back and advised that Opodo were not legally entitled to this amount. In October, 2021 the borders were opened and I notified Opodo that we would leave on 30/10/2021. They forwarded our eTickets to us with no mention of the £77. On our return to the Cape Town Airport on 23/11/2021 we were notified that Opodo had cancelled our return flights without notice on 3/11/2021, 3 days after we arrived in Cape Town. Emirates staff directed us to KLM Dutch airline to purchase 2 tickets for £820. We spent 15 hours in CT and Amsterdam airports awaiting our flights. I have tried to claim this £820 through my Credit Card Holder, thery say that they are not liable and that Opodo is at fault. I am unable to contact them as they do not use emails, only no-reply emails and telephone. Could you please help me? The email address is for the Opodo January Special Flights and are not responding to me. Opodo's sister-company is eDreams.