Woa up there horsey !!! A generator doesn't have neutral potential on it.
Both connections become live feeds and if it is to take over supplying the
house in the event of a power cut, then you'll need to upgrade the
earthing
and take only a single phase from the genny, to feed the house.

It works by making the grounding spike in to your neutral potential. Very
dangerous if not done properly. Not to mention the fact that, if the
mains
supply then comes back on while the genny is running, then it could blow
every fuse you have in the house and may even trip out at the local sub
station if it is a large capacity generator.

I originally thought that you had automatic change over, to bring the
genny
online when the mains failed. But you say "possible upgrade to an
automatic
system later". That's a big NO NO !!!!!

FYI A proper automatic transfer switch makes it impossible for both grid and
generator feed to get connected together!

Generally speaking, with a non synchronised genset, the grid fails, so the
ATS disconnects all power to the house, starts the generator, waits for the
generator to stabilise, then switched the house from nothing, to generator
power
If the grid is then restored, it will wait a bit, to make sure the grid is
staying on, then it disconnects the house, waits a bit, then switches over
to the grid (If the grid and genset are not synced then switching directly
would be messy!)

It then keeps the generator running for a bit, then finally shuts it off

I understand where you are coming from, as I wouldn't want to try to supply
power to my whole road! or kill someone working on the street wiring by back
feeding (I presume a step-down transformer would work in reverse, ramping my
230v up to 1000's!)