thanks for all the comments and advice guys, really helpfull
French and German clean air permits - I bought both just in case, didn't need the French one but definitely needed the German one for where we were staying in Stuttgart. Use the official sites, RAC has the right one for the French one, but use the Berlin site for the German one (EURO 10 cheaper).
Route in - took the lower route via Luxembourg, weather was terrible full on rain and really dense fog most of the way then Friday rush hour traffic at the end to top it all off, not a route id recommend. 9 hours calais to stuttgart
Return route - via Brussels, weather was better and daytime, so made up a lot of time on the Autobahns, roads were a lot nicer but more speed camera's in the Belgian side. 7 hours Stuttgart to Calais.
So Friday night was rest night then over to Affalterbach in the morning for the AMG Private lounge xmas gathering
Weather was still pretty terrible that evening torrential downpours but managed to meet up with a few MBClub members for a bite to eat.
Next day we braved the German public transport, single day ticket and ride on the S-Bhan trains to the main Stuttgart station, then another train to visit the Porsche Museum. Definitely worth a visit, we took a walk past the Porsche factory too.
Stuttgart xmas market is also worth a visit, we went there in the afternoon by train once more so I could have a few mugs of Gluhwein.
final day we had our own factory tour which was amazing, the British flag laid out especially for us, and got to park next to some Legends. The use of Ai to monitor the builds of the engines and control of the tools was so technologically advanced and the Factory full of GT Pro 2's was mind-blowing . (no pics allowed though in the factory which was a bummer!)