I post here in the hope of (finally) receiving an answer to my request.
I bought BTC via Blockchain.info using the sofort deposit method on March 20. Before proceeding to the transfer, Blockchain displayed the amount of BTC I was promised to receive for the amount of my transfer.
The next day, when they credited my BTC address, they used that day exchange rate and not the amount that was promised.
This is a difference of about 1.1 BTC
I used their support system the same day (March 21) to open a ticket which hasn't received an answer yet, and piuk remains silent to my PM on this forum.
It seems I was not the only one who had that problem as you can see on
StackExchange.