Hi Hunkydory,
Employment law states that an employee must give one weeks notice. That said, there is no redress available to the employer if you give less. I.e. under employment law there is no retribution to your employer if you give less than one weeks notice.
However, you have signed a contract with 8 weeks notice. your employee in theory could, under contract law, hold you to that. However, if you dont give 8 weeks, your employer would have to sue you through the courts under contract law and this would be costly. Generally speaking enforcing contractual notice via the courts only happens for top executive......as the costs arent worth it.
hope this helps...