When you lodged it the exchange rate on the day was applied and the euro amount credited to your account, now that it has gone through the system and been returned unpaid the debit to your account will be at the rate on the day of debit and as sterling has got more expensive this probably means that you have suffered a loss on that part of the transaction firstly, then there is the processing fee that the UK bank levied for the unpaid and finally the CU fee for a bounced cheque.