Piers,
I don't know if you have received any information on this, but I can tell you it is a bit of job I'm afraid. I believe there's welding involved! That's enough to scare me right into the shop of a professional[:0]
Most of this group does mostly mechanical repairs, and major body work is not something the average 113 owners does, and if they do it, perhaps only once
To get an overview and possible pricing, you should contact K&K; they supply a lot of sheet metal and might have an idea of what it takes to solve your problem. My own experience indicates you should forget the email and call them directly; you will end up speaking with Cheryl, the queen of sheet metal!
You can reach them at
http://www.kkmfg.com/ Tim Kidder is great guy, and they supplied all the sheet metal (don't ask!) used in my restoration. Best of luck!
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored