I decided to do the driver side seat base next as it is the back that is covered in the original leather that I need for reference in the future.  I started as before by grinding it down using an angle grinder with a wire wheel attached.  Because it is so noisy I kept the garage doors shut and the picture below shows how my much dust it produces.

It was more rotten than the passenger side base and it had been repaired before whereas the first base hadn't been.  I guess it has seen more use leading to more wear?

I made some sections to weld in.

I then gave it a coat of paint and because it was late in the afternoon I hung it in the sun to see if it would dry enough for me to be able to ally a second coat, no luck.

I took the Gerry Anderson book out again and discovered that the base is only covered on the exposed edges and that means the vertical sides and I have enough material to do that which is good.