This is currently considered to be a specialist area, and the earthing can get complicated depending on the type of system and whether there is sufficient information available from the manufacturer. The IET has published a specific Code of Practice for this: IET Code of Practice for Electrical Energy Storage Systems.

The independent output is not permitted to rely on the distributor's means of earthing in the UK - this is covered in BS 7671 (Regulation 551.4.3.2.1). You will need a consumer earth electrode, but this can be connected to the MET.

The IET Code of Practice for Electrical Energy Storage Systems recommends such a “backup supply" operates in TN-S in island mode. Again, requirements for transfer of supply and switching means of earthing must comply with a number of specific Regulations in BS 7671.

Will you be connecting it as a “switched alternative” to the mains for? If so, you need to comply with the requirements of G98 / G99. The detail of the required switching mechanisms are contained in the IET Code of Practice. Some “inverters” have the management system and switching (backup isolator and N-E bonding relay) in place to achieve this in a compliant manner. 

After having a quick look at the manual you posted a link for, I don't think there's sufficient information in there to tell me whether it has an appropriate switching to achieve backup earthing as required in the UK, and you'll need more information or a more detailed schematic (unless you have that already).

If the inverter “backup” output is “floating” ("earth free" or “IT”), it should only be used for a single piece of equipment that is not connected to anything else via communications cables etc. The IET Code of Practice does not recommend that a “floating” or IT arrangement “backup” output is connected to part of a fixed installation.

There's also the consideration of a suitable location for the batteries and inverter, and specific recommendations in BS 5839-6 relating to linked fire detection and alarm if inverter and/or battery are in a location that is not frequently visited in the premises. 

This is currently considered to be a specialist area, and the earthing can get complicated depending on the type of system and whether there is sufficient information available from the manufacturer. The IET has published a specific Code of Practice for this: IET Code of Practice for Electrical Energy Storage Systems.

The independent output is not permitted to rely on the distributor's means of earthing in the UK - this is covered in BS 7671 (Regulation 551.4.3.2.1). You will need a consumer earth electrode, but this can be connected to the MET.

The IET Code of Practice for Electrical Energy Storage Systems recommends such a “backup supply" operates in TN-S in island mode. Again, requirements for transfer of supply and switching means of earthing must comply with a number of specific Regulations in BS 7671.

Will you be connecting it as a “switched alternative” to the mains for? If so, you need to comply with the requirements of G98 / G99. The detail of the required switching mechanisms are contained in the IET Code of Practice. Some “inverters” have the management system and switching (backup isolator and N-E bonding relay) in place to achieve this in a compliant manner. 

After having a quick look at the manual you posted a link for, I don't think there's sufficient information in there to tell me whether it has an appropriate switching to achieve backup earthing as required in the UK, and you'll need more information or a more detailed schematic (unless you have that already).

If the inverter “backup” output is “floating” ("earth free" or “IT”), it should only be used for a single piece of equipment that is not connected to anything else via communications cables etc. The IET Code of Practice does not recommend that a “floating” or IT arrangement “backup” output is connected to part of a fixed installation.

There's also the consideration of a suitable location for the batteries and inverter, and specific recommendations in BS 5839-6 relating to linked fire detection and alarm if inverter and/or battery are in a location that is not frequently visited in the premises.