MarkWilliams I'd just call Givenergy and ask for help or at least an explanation. According to the datasheet, the gen1 HY5 inverter can operate 180-280Vac, handling the PV and Battery DC side appropriately.
Our grid voltage is within spec (230V +10% -6%) usually 242-248V, but our Zappi regularly trips on overvoltage of 258V+ and the DNO isn't interested unless the voltage exceeds 264V! In the olden days (pre EU) UK voltage was 240V +10% or -10%. Unfortunately, myEnergi are saying that their kit is working correctly when the voltage exceeds 230V+10%, it trips. Givenergy are saying that their kit is working correctly, simply raising the voltage by up to 3V to export up to 3.68kW and the DNO is saying that the local distribution system is within spec at 242-248V. Anyway, you may be able to get your DNO to reduce your local voltage.
(Rant over) I would expect a Grid Voltage Fault to be caused by a disconnection rather than an out-of-range condition, but Givenergy will be able to advise.