Brian,
From what section and page did you find the instructions for replacing the distributor? I have an original hard copy of the shop manual and cannot find those instructions.
You won't.... that particular (and useful) page isn't included in the TC Europa manual but it is in the Elan manual.
I have no idea why they didn't just copy the Elan Engine section across to the TC Europa manual (it is the same engine after all) but they didn't, preferring instead to re-write them. As the Europa manual section has fewer pages than the Elan section I can only think it was Chapman again on his quest to "add lightness".
Edit to add;
IIRC it's a 7/16" spanner you need for the bolt holding it to the block and you can get away with a very small one as it doesn't need a lot of tightening. About 3" long and you'll find it easier to work with the smaller length.
Now an admission - my luggage tray isn't bolted in place, it merely rests on the body/chassis and so it is very easy to slide it out from the rear of the car. At that point you can step into the engine bay, sit on the gearbox and the access to the distributor via the rear carb is probably the best you'll get. Heck, you can even see what you're doing !
It sounds mad, but it's far easier than stretching over the side of the car and trying to work by feel from above the carbs.