Yep the sensor is tricky to get to but not impossible. Find it from above but remove/replace it from below. It is right below the top radiator hose on the left hand side of the rad. (Left as you sit in the car - same side as the fuse box
) It will have a short cable leading to it. Wiggle the plug off the sensor and you will see a grey plastic 3 pin electrical terminal with a 30mm brass hex base. You will need to buy a 30mm deep reach socket to get it out. It is screwed into plastic radiator so low torque so you don't need to buy a posh brand. Just get the cheapest socket you can. You won't need a 30mm deep reach that often
If you can't find it, I will go and take a pic of it for you but that is a pretty fool proof description above
There are multiple threads on how to replace the switch most involve removing fans and/or top hoses. I only have experience of 16V cars. On a 994S my recollection is that I removed airbox and top rad hose to get to it from above. On S2 I got it out from above without removing anything other than skin from my knuckles. Couldn't get it back in form above though so had to jack up car and take of the engine tray and come in from below without removing anything else.
If I was doing it again I'd go straight in from below for removal and replacement. Have the new one ready to pop straight in so that you don't spill too much radiator fluid. Make sure the motor is not too hot because you may get a green shower!
When it is back in bleed air out of the system as per Clarkes garage instructions .. not difficult and do it before you take car off axle stands. Oh ... last thing ... don't forget to put the fan relay back in!!
Let me know if you need a pic. Good luck!