Here are a few photo's showing how I reached the Spark Plug. It took me all of 10 mins to change the plug. It's not very difficult to reach it, and you can't see the plug at all, just the plug boot and wire. Removing the plug itself is by feel. It is also a good idea to use some compressed air to blow away any dirt and debris out of the SP recess after you remove the plug boot. That way nothing will fall into the cylinder after the plug is removed.
Just reach around the cylinder head and grab hold of the boot, pull if off.
My finger is on the wire/boot. When you pull it off (it may take some wiggling) don't pull on the wire itself, grab a hold of the boot.
Use the plug wrench from the Vespa tool kit, it is much easier and was made to get into this tight spot, the open end wrench works best too.