Replacing the Spark Plugs on a 928 can be tedious sometimes. Especially the #1 plug is hard to reach as there is little room to work in. I guess everyone has his own solution, but this is mine:
It consists of a deep 21mm socket that fits the spark plug. I don't use a magnetic one or with a rubber insert.
The next is two short 3/8inch 5cm long extensions. Not one long one as that creates a problem inserting and taking out. using two short ones stacked on top of each other enables you remove the first extension while the rest is still in the head. It uses less maneuvering room this way.
On top of that I use a wrench with ratchet. This is one that can wrench on an angle, so it is easy to move it into an area where more room is.
Last but not least: I use a grabbing tool that holds the spark plug. It makes removing and inserting sparkplugs a lot easier. If I use a magnetic socket or one with a rubber insert, I kept separating the short extensions or the socket all the time. By turning the plug with the grabbing tool a few turns I also assure that it goes in correctly and not damage the threads in the head.