This morning, I took the cover off the car, and was greeted by the same stench as yesterday. He'd come back.
So, I moved him to a bit of land behind my garage, and started work.
I took the driver's seat out, and stated to make some new brackets. I decided that I needed to make the seat moveable (to some extent).
And after a lot of cutting, welding, grinding and cooling, I made the brackets.
There's a 50mm thick section of foam behind it - I'd done some measuring with the help of my neighbour's daughter's boyfriend to see where the best place was; a 100mm piece of foam was used for this test - it didn't seem to uncomfortable. I then used the thinner foam, and adjusted the seat to the rear-most position.
This now has enough space for the passengers to sit, and also a reasonably comfortable ride for the driver. Assuming he's 5ft 11, like me. Anyone taller may find it a bit cramped...