a decision to drive when fatigued puts you in the wrong, not your employer.
However, the issue of vicarious liability makes the employer responsible for the actions of the employee whilst engaged in work based activities. A certain amount of personal culpability would fall on the employee's shoulders but ultimately the responsibility is the employers.
I'm not saying that I agree with it completely as I don't feel that employers can or should be expected to lead adult employees by the hand with everything they do - they have to accept responsibility for their own actions - but the legal fact is that the employer bears the responsibility.
This is being pushed to the extreme in certain areas; in the USA an employer was recently found to be responsible for an employee drunk driving home from a work sponsored social event, outside normal working hours. How long do UK cases take to follow those in the US???
Also, on the subject of hire cars, the employer has further duties under the Provison and Use of Work Equipment Regulations - if the car was in any way defective the employer would be responsible, as well as the hire car company.
Now I\'m starting to lose the will to live.