I had a similar issue recently with an online retailer.
I followed two tracks to resolve it.
First with the retailer who was happy with a screen shot of the online statement together with info on the authorization number, date, time of the transactions. My telephone banking people could give me this over the phone.
Thankfully, they refunded me the transactions without much hoopla, so I did not need option two, which was;
REporting the transactions as fraudulent to your bank.
If Tesco have agreed to provide you a refund (have you an email or letter confirming this and the date it was actioned?) the bank will chase it up on your behalf with the Merchant Bank that processed the transaction.
Again, my telephone banking folks were very good with info on this.
Best to call your bank, tell them the story with dates and time and ask their advice though.