After a week with a cold, and doing nothing, I felt better today so I stripped it down and had one more go. I found that the old Imperial 1/4 in socket set along with a 1/4 in socket and the hex extension from my DeWalt drill/screwdriver, just fitted in. And with a short extension on the handle and some thick leather gloves, the screw gave way!!
The CPS is swapped, but I'm not confident. The replacement is a Bosch which I've carried as a spare for a long time. The car struggled to start. It did eventually get running. And then it restarted OK. Stopped for lunch now, and will try again later, and see how it goes.
In the meantime I bought a spare CPS from a Saab specialist. They only offered two. A "high quality" unnamed one and a Valeo one. I bought the Valeo. It was just going to be a "get you home" but I may put that in instead.