Hi Phil,
Welcome to Aero ownership:) Never had overheating issues on my S1. Sure the sills get hot but that's just the design. The hot air blowing into the cabin is pretty normal, basically heat soak from the engine. The air con in my experience did little to fight it. What you may notice is with the roof and windows up the heat soak reduces a bit and the aircon improves. I put this down to less air being drawn into the cockpit with the roof up.
Best option is roof down as much as possible to stay cool.
Check engine light could be the remap, could just be a sensor. Get a garage to check the error code and reset it, should give you a decent idea.
Fiery I can't offer mOre help, sure a current S 1 owner might be able to add more.
