Peter, I had an identical failure about two months ago. There was no way that the sheared bolt could be removed with a stud extractor, believe me I and my local garage tried and they even heated the de dion with oxy acetalyne to loosen it, all that happened was the extractor broke. We came to the conclusion that the heat generated in the shearing bolt had effectively welded it in.
The shock thrashed about hitting the half shaft but luckily the clip holding the cv boot on was dislodged but the rubber wasn't pierced. I just refitted the clip.
The cause in my case was an overestressed bolt which had been cross-threaded in a previous shock change. I remember the garage told me they had to tap a new thread.
It's likely given the age of yuor car, the same as mine, that your de dion has the old style central hole which I used to refit the shock.You need a long 1/2 inch unf or unc bolt and nylock nut for both sides.
Not only was the car driveable it's been like that ever since. The ride height was adjusted back to normal but you don't need to do this to get home.
E-mail a contact number if I can be of any help.