A double pole, double throw relay can do the switchover; that's the easy part.
The issue is how is the the second load sensed.
"I want to take a supply of AC power which will be nominally 240V, single phase , 10A.
I want to have it going to a load until there is demand from another load which can be sensed when a 240V line becomes active as it will have another supply. This must be isolated from the other load until it has changed over.
When that other load fires up, I want to have something ( simple as possible) that will disconnect the first load and direct it's power to the 2nd load.
When the second load switches off, the power will go back to the first load."
If the second load going on results in a 240AC signal with enough current to activate the 240V relay coil of a double pole, double throw relay, then that is all you would need. You would have to pick up this "active circuit" signal from somewhere internal to the appliance... it would drive the coil of the relay, which would switch the contacts to the new panel supply. The contacts are open before change - there would be no contact between the two loads, ever, even if the relay fails. The relay coil takes very little current and has no connection to the load contacts whatsoever.
I'm guessing that the power being switched is the grid tie inverter output going to another panel???
Alternately, if the second (highest priority) load is something on a different panel, and you are trying to shift your grid tie inverter ouput to cover it, it can be sensed with a current detector switch added to that circuit after the breaker. That switch can switch the double pole relay coil current on so that the inverter output is now applied to the second panel.
If you can be more specific about your design concept (a rough sketch perhaps?) I'll try to help you come up with something specific with a wiring diagram to smoke test.