Hi

Having owned a firm of Mobile Mechanics....I know you will find one in your area or any area for that matter....
But for a Porsche ...I wouldnt....unless it was for very general stuff like an oil change or brakes...also you need to ask them if they have Public Liability Insurance...its VERY VERY expensive for Mobile guys and most do not bother...they MUST also have Motor Trade Insurance specifically for Mobile Mechanics....that covers them to drive your car which is even more expensive....
Ask to see BOTH certificates when they arrive.... !!!
Hope this helps...
All the best Brett
