In future when paying ask them to email the receipt. It's an option on purchases.

Although COT'ing can sometimes look like the best option in a lot of cases it can go wrong. Before you even consider doing a COT you need to credit check the new business or sole trader that is taking it over, if it fails credit and the transfer falls over you're going to be stuck with BES and now you're going to be on deemed rates. Essentially you're going to be in an incredibly poor position price wise and will end up having to agree a contract to get off deemed rate, you can almost be sure it's going to higher prices than you tried to COT away from. Tread carefully. Raise a complaint. Don't just moan about things, be accurate about what happened, don't leave details out, and tell them what you want, tell them you want releasing from the contract and returning to your previous supplier by ET. Contact your previous supplier and ask them to help. Raise a full subject access request. Request the recording. Challenge anything that isn't what you agreed. Get the complaint to 8 weeks and BES should automatically notify you that it's being escalated to Ombudsman. If they don't contact the ombudsman they are in breach here. Worst case scenario is you're stuck with BES but you've caused them trouble and cost them money with the ombudsman. Best case you win and they let you go.