It did work from the engine alternators or the new 12v genset through the inverter but it was just not efficient and also an engine had to be running to use it so noisy too. So reluctantly I have had to admit I was wrong. I know I was told many times on the forums and had quizzical raise eyebrows from people I spoke to about it face to face.
So I now have a gas oven and full width grill and jolly well (and silently) it works too.
Once fitted I did a gas leak check with a digital manometer. Start pressure was 39mb. The supply was turned off and the pressure dropped to 30mb by turning on a gas ring. It was left to settle for 5 minutes where it dropped 4mb. After another 5 minutes the pressure had risen slightly to 28mb. I left it another 20 minutes or so which is more than is required and the pressure remained at 28mb. Test passed.