yes - because of being blocked in at a lower level on a NATS track and once from the eastern med going westbound when we couldn't make flight plan level and were left at FL290 for several hours.
Also had it one time east bound where we were out of the narrow jet stream core and lost 60+knots of chuff wind.
Always worked on 4% per hour for the extra fuel tanked - take 10 tons extra fuel and at destination after 10 hrs you have 6 tons left.