OP
spider
(however it is based on a floating earth)
As your system is a TN-C-S system even when the main switch is open the earth is still connected.
If you connect an additional earth to the star point of the generator (common for island mode operation) you are in danger of introducing circulating currents into the system which could overload current caryying capacity of your main cabling.
As previously postedy others the safest and best way of connecting a generator is via a change-over switch (either manual or automatic)
If your generator is not expected to pick up the full load of the property you could fit undervoltage devices to all non essential circuits, that way when the mains fails the circuits will automatically disconnect; when the generator runs up to speed and closes onto the bus bar only essential cisrcuits are still connected.
I propose to install a seperate earth rod connected via a 16mm conductor to the consumer units earthing bar, the single phase generator will have the neutral/earth tied at the local socket inlet for it.