Well I have worked it out! So since I posed the question I had better tell you the answer!
I have found a photo on the internet of one installed in a boat – in the USA I think. It satisfied me that there was no tray or floor underneath the oven. So I suspected something had come apart that should not have. Anyway I took the front strip which contains the control dial off. It was held on by 4 pop rivets so it was easy to drill them out. This is the result:
The tube in the centre of the picture is the burner nozzle.
Now when you look at the carrier unit:
When you look closely at the hole in the centre of this picture you can see 4 dimples, just on the outside of the holes upturned flange. They are actually the remains of spot welds. When you turn over the component in the last picture you can see similar marks. Originally I thought they were dimples to provide a gap for air to get into the burner. Having confirmed that there is no missing floor, I thought again and looked more carefully.
So this is the burner unit held together for welding:
So now I need to buy some stainless welding wire and weld the two parts back together. Then it will just remain to rivet the complete unit back into position.
So problem solved!!!!
In case you are curious I did get a partial refund on the purchase price to reflect the fact that it was not in the “excellent as new condition” that was claimed. I didnt want a total refund (which I was offered) and to give it back – I knew I would be able to repair it when I had worked out what was wrong with it.