RULE 4 - PERFORMING A PRE EXISTING DUTY CANNOT BE CONSIDERATION
2. A duty imposed under an existing contract with the promisor
Stilk v Myrick - 3 sailors refused to work the journey home so the others tried to claim additional payment. LEGAL PRINCIPLE - Making boat safe and seaworthy part of pre existing contract
Glasbrook Bros V Glamorgan County - police provided protection at minors strike per request of mine owner. They had to pay extra as the police force provided extra protection.
Hartley v Ponsonby - 1/2 crew deserted - there was consideration, entitled to extra pay.