Thanks everyone. Our return flight is from Barcelona with Vueling. They are under different booking references, so completely separate. We have already checked in for our Vueling flight. We can't check in for Lufthansa until Sunday. So I think we will be OK there....
I'm just concerned that FbC will be "worried" about us not showing up, and assuming we have missed the flight. Incidentally, we have received zero confirmations of our flights from Celebrity. I have emailed their flight department, but got absolutely nada. Thankfully, I noted the booking references when I made payment.
@rslrhu How are we supposed to let them know on the day? They take over an hour to answer the phone! A complete time-waste!
@doktor71 Yes, we have been through Frankfurt for a previous cruise, but booked flights ourselves for the day before. We missed the connecting flight to Rome, so stayed in Frankfurt overnight. No problem really, just had a super early flight to Rome the next morning. I would have been happy to change this flight, as I just feel it's too risky. Our itinerary means that we would likely miss the first 4 days of the cruise, and it's just not worth the worry. I'm happy with the Ryanair flights, and if there are any problems, our Plan B is still the Lufthansa option.