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 with a rubber insert to grab the spark plug while taking out or inserting. Important is that the rubber is seated properly and does not separate from the socket.
The next is two short 1/4 extensions. Not one long one as that creates a problem inserting and taking out. using two short ones stacked makes you remove the first bit while the rest is still in the head. It uses way room this way.
And the last bit is the 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.