Not to plan
I decided to only extend the one side as I had troubles with warping, and it was the stronger supported side of the safe. I'll explain. To extend the safe, I cut the left side panel away and welded 2 panels, and a top and bottom in between, then welded the side panel back on.
This became a bit of a nightmare when it came to lining up the panels and keeping them straight, especially on the small strip of steel that frames the LHS of the door, and this strip was stronger than the RHS due to it being an "s" shape, where the RHS is just a flat piece of steel around 40mm wide.
The LHS warped easily and I had a couple hours of pulling and pushing the door frame after I welded it all up just to get the door to close (I also welded some extra supports inside to reduce warping). I figured the RHS is going to be a slightly more difficult job, and I don't have the time at work to do it anymore. I'll have to bring it back at christmas
In hindsight, I should have cut a single piece of steel large enough to encompass the front, side and rear in one piece, instead of 3 separate panels. That will make it easier to paint as well.
When I fill this side up, I'll likely pull it out again and finish the job. But for now, it'll do.