Thanks.
I've spoken to the dealer as well as Mercedes technical and Sachs, the manufacturer, and they can offer no advice. Sachs did tell me that it was a "captivated market part", that is that it is only available through Mercedes.
I too was beginning to wonder if there was some kind of rebound spring inside the shock and so decided to re-assemble the spring onto the strut (with some difficulty due to the short length) to see if it then extends.
The result is inconclusive at the moment because we only have one strut off the car but the newly assembled strut is still shorter than the liberated old one. Tomorrow we'll remove the other strut and do a side by side comparison.