Contract Law Exam Q.

Hi I am trying to answer this past exam question. Could anyone give me tips or brief answer of what to consider please?
5. You are a legal adviser working for Jack Smith who operates his own garden centre business called Jack’s Garden Centre. At Christmas time, Jack advertised that all customers spending £10 or more between 1st December and 24th December would be given a free gift. On 14th December Jack realizes it is costing his business too much to give away free gifts, so tells his employees not to hand out any more free gifts. Several customers complain and demand a reduction in the price of goods bought to compensate them for lack of a free gift.
Using cases, statutes and your knowledge of contract law, explain Jack’s rights and obligations in these circumstances.
Many Many Thanks

Steve