To add to the various posts on here regarding methods of changing the crank sensor on a 156, I thought I'd share my experience and, hopefully, a useful tip.
After trying to reach it from the top and bottom several times (Throttle body and battery off) and failing miserably to reach the thing, I decided to give it one last go from underneath before admitting defeat and booking it in to a garage.
However, this time I armed myself with the little tool I found on fleabay
If you've got a large-ish allen key already, you may not need this, but the one in my set was a bit too small to provide the necessary leverage to undo the bolt.
Having got the car up on my halfords car ramps I found that there was just enough of a gap in the subframe to the left of the sump to get my arm up and place the end into the little socket head bolt holding the cps (providing you don't have forearms like Popeye!). Then, using a small 10mm ring spanner, was able to slip it over the end of the tool and gain enough leverage to crack-off the bolt. After this, it was fairy easy to unscrew it. A torch strategically placed beside the sump might have helped to provide a little more light, but I could just about see enough with the bonnet up, though you do strain your eyes a bit trying to see.
Once I'd managed to push in the new cps, after clipping it in and dangling it down the engine, I put a little blob of grease on the end of the allen key tool and place the cap head on. The grease helped to hold it while I carefully positioned it back in it's place and hand-tightened it. It was then a matter of using the ring spanner to tighten it further a quarter of a turn at a time. It was a bit fiddly and dropped down a few times, but with patience I managed to get it tight. Such a relief and nice not to worry when the car would decide to stop again. I was also able to do this without having to undon the exhaust but, again, bulging forearms may make the job more difficult for some!
Hope this helps anyone who, like me, hasn't mastered the art of changing them from the top of the engine.