You'll still need to make sure that the genny is earthed properly.

Out of interest, what would you need to do if you have PME earth?

You would obviously need an earth rod and 100mA Type S RCD arrangement.
However, do you have to disconnect the supplier's earth upon power failure?
If so, presumably it is simpler to just use the TT system at all times, or
is there a way of dealing with this?

Can you assume that there will be nothing unduly bad on the PME earth and
leave it connected through?

Christian.