AR??????? LOL - please elaborate for your UK buddy....
100C coolant is fine, why would you want to reduce it? Consider that the optimum temperature of the automotive combustion engine (not F1 car etc..) is around 95C, so under a little stress I would consider 100C fine.
Others have reported 120C before the VFC cuts in - this is poor as the Aux fan operates before! Then action is required - ie take recommendations from Jim and others...
With my VFC I can tell engagement condition from inside the car as it's real loud. But on my 190E 2.6 I couldn't! So the only check is by rpm for your car I'm afraid!
Lea