You can check Historical Reference Rates on the ECB exchange rates page.
To find an historical rates, scroll down to the ‘Historical reference rates’ section on the Euro foreign exchange reference rates page.
Using the calendar provided, you can locate the historical date required and download the exchange rates as part of a PDF.
Historical Mastercard currency conversion rates can be found on the MasterCard Currency Converter page by selecting the ‘Select date of transaction’
To calculate the percentage difference:
Mastercard Exchange Rate (Including KBC fee) 1 EURO – 0.906897 GBP European Foreign Exchange Rate = 0.88460 GBP
Difference = 0.022297