You are quite correct mike there has to be consideration (unless contract is by deed). To create a valid contract there also has to be offer & acceptance, an intention to create relations, capacity to contract and of course it must be legal. But I suspect Chris is aware of this, you may be interested in the well-known case of White v Bluett 1853
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The European courts of justice, following an appeal by a Spanish trade union, ruled recently that if you are on holiday and become sick you could take your holiday entitlement at a later date when you have recovered.