Just to update on this. In the event the crew were very informative on the reasons for the delay: the original aircraft (G-XLEE) scheduled for BA 055 on the evening of 17th burst a tyre on landing at LHR from LAX that morning, and tyre debris damaged the aircraft fuselage skin and also a window, and had to be taken out of service. The only possible replacement (G-XLEF) could not be made available until the following day, hence the 15 hour delay in departure.

Much time was made up at JNB but eventually we landed this morning 330 minutes late (flightstats.com) so it seems clear to me that there were no allowable exceptional circumstances, and my 261 claim will be for Euros 600 for each of me and Mrs Charlie Whiskey.