Carol had entered into a contract to design and build a pergola for Janet's garden for the sum of £800. Although she was quick to produce plans and drawings, she still had not purchased the necessary materials after three months. Janet's mother, fearing that the summer was likely to be over before the pergola was in place, offered Carol an additional £100 to build the pergola. Carol completed the pergola. Is she entitled to claim the £100 from Janet's mother?