European airlines don't really do stand-by or SDC. Everything is fare-dependent, so if your fare is flexible you can change to a later/earlier flight. If your inbound flight is late and your next flight has already left, it's a case of too bad, so sad. You'll probably have to buy a new return ticket. No one can predict whether your arrival will be on time or two hours late. These are the risks you take on board in separate ticket scenarios.