Before leaving I had taken the opportunity (as encouraged by the airline) to check in on line. Only to be told at the check in desk that I had wasted my time as self check in is not recognised at TFS.
Must be something to do with your flight originating in the UK. I fly from Bilbao to TFS and back regularly and always check-in online. Never been questioned at the check-in desk, just show self printed boarding pass, leave my baggage and go to departures. When boarding I always show my passport opened at the ID page as a matter of courtesy and to avoid lost time embarking.