Looks like this is the London end of the string of incidents to Athens flights I described above.
The 777 return to base appears to have been due to an engine surge.
I wonder when upper management finally clicked in, overrode Alex Cruz's "save money every day" blandishments, and managed to operate what then seem to have been a whole string of extra Athens flights late on the Wednesday and into Thursday to sort out the backlog.
I wonder if the BA statement "we provided refreshments" was what I got at my last delay at T5, a £5 (yes) voucher, which not only would not cover a coffee and a pastry but was airily declined when I presented it at the Pret-a-Manger right by the T5 domestic departure gates with "oh, we don't accept those". Wherever else are you meant to spend it ?