Using this Which? article as reference. You have up to 3 or 4 weeks to outright reject an item if faulty for a refund. After that period, up to 6 months, it is the responsibility of the seller to repair/replace the item or prove the buyer caused the fault. After 6 months the situation is reversed and the buyer has to prove the fault existed at the time of purchase, which may entail an expert's report supporting their claim; this can be claimable for up to 6 years. In my experience it seems most shops give a 1 year no-quibble guarantee against faults.