Well, you need to set the variable $_SESSION['currency'] to the right value.
So basically you need to have GBP and EURO enabled. THis would normally mean that the user could choose between the two. When they choose it sets $_SESSION['currency']. Instead of letting them choose you force that variable to be what you want.
If that is all greek to you just add this line
Code:
$_SESSION['currency'] = 'EUR';
At the bottom of the file includes/init_includes/init_currencies.php. It needs to go just above the '?>' at the end of the file.
And you need to check that you have euros enabled.