It would seem to me, that in order to provide an earthed neutral in island mode, WITHOUT use of the suppliers means of earthing, that this unit requires an independent means of earthing. Such as a length of green/yellow cable to an earth rod. Is this installed ? I suspect not.

I would expect FOUR incoming wires, phase, neutral, and earth, probably combined in a three core cable, like any other class one appliance, AND an additional earth wire to the earth rod. The two earth connections should be permanently connected to each other within the equipment.

In normal or grid connected mode, I would expect that the outgoing neutral, to the loads is connected to the incoming neutral. The incoming neutral is earthed elsewhere by the DNO, and in grid connected mode NO ADDITIONAL  connection between neutral and earth should be made as this is contrary to UK regulations.

In island mode the incoming neutral and phase conductors should be disconnected by a relay or other suitable means. The earth wire might remain connected but is not to be used as the means of earthing, or it might become disconnected. The outgoing neutral, now isolated from the incoming neutral, should now be automatically earthed by connection to the earth rod. 

In grid connected mode, this means that the unit has TWO earth connections, one via the earth core core in the mains supply cable, and a second one via the independent earth wire to the earth rod. This is fine,and  no different from a water heater earthed via the green/yellow core in the supply cable and again earthed via metallic water pipes, or say a central heating pump.

In short, multiple connections to earth are fine and often occur fortuitously. The neutral conductor must be earthed, but ONLY ONCE either by the DNO, or in island mode by the customer, NEVER BOTH at the same time.

It would seem to me, that in order to provide an earthed neutral in island mode, WITHOUT use of the suppliers means of earthing, that this unit requires an independent means of earthing. Such as a length of green/yellow cable to an earth rod. Is this installed ? I suspect not.

I would expect FOUR incoming wires, phase, neutral, and earth, probably combined in a three core cable, like any other class one appliance, AND an additional earth wire to the earth rod. The two earth connections should be permanently connected to each other within the equipment.

In normal or grid connected mode, I would expect that the outgoing neutral, to the loads is connected to the incoming neutral. The incoming neutral is earthed elsewhere by the DNO, and in grid connected mode NO ADDITIONAL  connection between neutral and earth should be made as this is contrary to UK regulations.

In island mode the incoming neutral and phase conductors should be disconnected by a relay or other suitable means. The earth wire might remain connected but is not to be used as the means of earthing, or it might become disconnected. The outgoing neutral, now isolated from the incoming neutral, should now be automatically earthed by connection to the earth rod. 

In grid connected mode, this means that the unit has TWO earth connections, one via the earth core core in the mains supply cable, and a second one via the independent earth wire to the earth rod. This is fine,and  no different from a water heater earthed via the green/yellow core in the supply cable and again earthed via metallic water pipes, or say a central heating pump.

In short, multiple connections to earth are fine and often occur fortuitously. The neutral conductor must be earthed, but ONLY ONCE either by the DNO, or in island mode by the customer, NEVER BOTH at the same time.