Fortunately this end of the door is pretty flat so clamped it to the table to stretch the weld out flat. Bit tricky under the lip so just slid my hammer along while hitting it with the mallet. Nice and flat now without any distortion left.
Cleaned up ready for the next stage. Folded up a new piece for the replacement frame. Made from a single piece but welded where the end folds came together.
Checking for fitment. Need to put the drain back in again otherwise this repair wont last long. Using a 1mm cutting disc I cut into the corner and along the front edge.
Positioned my stake dolly on the edge where I wanted the fold to go and hammered it over just using a small square faced hammer. The ends just need a bit of weld which I did with some copper in the slot to stop it filling the front.
The door skin and the back of the new piece was zinc coated before fully welding it in and planishing it flat. Front face gets coated and backside of the flange to be folded.