Normally booked well in advance on inflexible tickets, but on one of the few occasions i was on a full fare J class (to CPT) i found out exactly how useful it was to be fully flexible. Had to change the return date several times whilst i was in CPT as we werent sure whether we were going to finish the project on time. Extending it by a few days, then shortening it by a few. Eventually on the final day we finished up much earlier than expected (after being away for 3.5 weeks) and out of curiosity i called up to see could i get on the late departure back to London that night. A very helpful young man said yes, you certainly can as we have a seat and you're on a fully flex ticket. 4 hours later i was sat onboard with a glass of cheap sparkling wine. The joy i felt at getting home for the weekend was worth the extra money my employer must have forked out