For the purposes of this Part an employee is dismissed by his employer if (and, subject to subsection (2) . . . , only if)—
the contract under which he is employed is terminated by the employer (whether with or without notice),
he is employed under a limited-term contract and that contract terminates by virtue of the limiting event without being renewed under the same contract, or
the employee terminates the contract under which he is employed (with or without notice) in circumstances in which he is entitled to terminate it without notice by reason of the employer's conduct.
An employee shall be taken to be dismissed by his employer for the purposes of this Part if—
the employer gives notice to the employee to terminate his contract of employment, and
at a time within the period of that notice the employee gives notice to the employer to terminate the contract of employment on a date earlier than the date on which the employer's notice is due to expire;
and the reason for the dismissal is to be taken to be the reason for which the employer's notice is given.