I was delayed 24 hours in May on a BA 787-9 from Singapore to Heathrow. We departed the gate on time but were still on the ground 2 hours later. An announcement was made that there was an engine fault and we returned to the gate where a "procedure' was carried out, which then entailed a 15 minute engine run at high power, so we left the gate to do that. It didn't cure the fault so we returned to the gate. The captain gave us the option to eat on board since we had been there for five hours by then. We were bussed to hotels and waited till a spare part was flown out from UK. We departed 24 hours later.