As hc4 says it is the sale of goods act 1979 as amended by the sale and supply of goods act 1994. Goods, which fail within the first 6 months, are found to have been supplied in an unsatisfactory condition and it is for the supplier to show they have subsequently been damaged, perhaps by being dropped or incorrectly used.
Satisfactory quality is defined in section 14 (2B) the word satisfactory was introduced in the 94 act prior to which it was merchantable quality a term of uncertain legal meaning.