Ended up trying the DC function and being pretty happy with it. Have decided to keep the welder, and not kid myself with the idea I can do ally with a cheap AC TIG.
So with that in mind, I remade the fuel tank out of steel instead.
I went with 2mm sheet, I know that's overkill however;
1. the tank makes up the floor in the boot, so strong = good, and
2. I had some available.
The welds are ok, considering I've never TIG welded at all before a week ago i'm pretty happy with them.
popped it in the boot to make sure it's all ok. Still have to pressure test it, then weld the filler, pump flange and earth on. Then I can seal the inside and paint the outside. But I know it'll pass pressure testing - after I finished welding it up it was pretty hot, and as it cooled you could see there was a vacuum in it because it sucked the top and bottom in slightly
Going to seal it with POR 15 once it's done. Also need to clean the shed... and the car...