What you want is an "Transfer Switch" either manual or automatic.
Rainbow Power sells an automatic transfer switch. But then you are int
the UK and Rainbow power are in OZ.
The switch must be a "Break before Make" type, what ever you choose.
This is the part from RPC. <rpc.com.au>
INX-L2-40 Automatic AC Transfer Switch 2 Way 40Amp Cost here is around
$400.AU
They ship around the world.