What you will require is the generator to provide an earth referenced neutral, so the generator star point must be bonded to an earth electrode in close proximity to the generator. If the star point was not earthed then an earth fault on one of the lines would raise the potential of the neutral with respect to earth. Since the generator casing could become energised it too must be bonded to the earth electrode.
Now you need to consider how to achieve ADS in the event of a short circuit and earth fault - the supply's and fault loop impedances matter here.
If the impedances are so high that insufficient earth fault current flows to operate the ADS such as fuses or mcbs quickly enough you have to employ RCDs to shut off the generator supply. In this event the generator and installation will become a TT provided that the installation MET is connected to an earth electrode. One should not rely on what the supplier has in place external to the building to regularly earth the neutral and cpc.
But since the generator and installation MET are in close proximity, and within the equi-potential zone the two separate earth electrodes can and should be one and the same to reduce the earth fault loop impedance and the touch voltage. Thus, bond the generator earth electrode and MET together. One cannot get a lower earth fault loop impedance by anything other than connecting the MET to the generator star point bonded to terra firma by an earth electrode. So TT becomes TN-S
If the EFLI are low then the generator and installation can form a TN-S, with ADS by fuses or MCBs and generator earth electrode and MET bonded together.
That's my
reasoning and answer. I have assumed the generator is within the footprint of the building or closely adjacent to it.
At all times the electrical wiring in the building must preserve the automatic and fast means whilst it is energised to detect and shut off supply in the event of an earth fault irrespective of source of supply.
See this too:
OC 482/2: Electrical safety of independent low-voltage ac - http://www.hse.gov.uk/foi/internalops/ocs/400-499/oc482_2.htm