Seemed as if I only needed to replace the lower 10 cm of the inner frame, as a new section of skin had already been welded in. And for a change, it looked quite well done. Not. The smallest application of heat caused buckling with a bump up to half a cm. I tried Jeroen's newly taught skills on it, but in the end I cut through the old weld because there wasn't enough room for any of my anvils on the sides, pulled the two pieces together, and have started tacking them together. Almost two mm's were cut out - probably way too much for my new shrinking skills to handle.
Most of the clamps already removed. NB door is upside down :-)