WESTERN EXCAVATING (E.C.C.) LTD V SHARP [1978] 1.Q.B. 761

If the employer is guilty of conduct which is a significant breach  going to the root of the  contract of employment, or which shows that the employer no longer intends to be bound  by one or more of the essential terms of  the contract, then the employee is entitled to treat himself  as discharged  from any further performance.