You wrote: "I was planning to put a leisure battery in, a friend has put one under the passenger seat. Any other suggestions?"
I built a box which I placed behind rear seats. The inverter sits underneath the seat. No idea where the original japanese inverter sits, but even if you could replace the original inverter, the cables for 100v might also need to be exchanged for the 220v. My inverter has two three pin plugs as outlets, so I just use an extension lead for easier access, Thats why I didn't place my inverter in the engine room, or underneath somewhere.