Welcome to the board Gunter.
I would take the car back and discuss it with your stockist. Any authorised Hornby stockist would be quite happy to discuss this issue. You might receive a free replacement motor instead of a replacement car however.
The same would apply no doubt to any Ninco, Fly, SCX or Carrera car purchased that suffered a similar issue and had not been fiddled with.
The real issue is for the stockist is that many experianced slot car owners start fiddling with their cars as soon as they are purchased, and as soon as you do this then you would loose any right of redress no matter how young the car is.
If you are known to your stockist as a fiddler then your chances of any redress will be limited.
However Gunter I do not believe this is an issue in your case and so the outcome of your discussions will no doubt be to your satisfaction.