"What does the guarantee cover?
Our guarantees provide full cover against mechanical breakdown through normal use. This means that if your item develops a fault during the guarantee period, we'll arrange for it to be repaired free of charge, including all parts and labour. Your guarantee is transferable with ownership, so if you give or sell the item to someone during the guarantee period, it's still covered."
"If we can’t repair your item, we’ll replace it with an item of equivalent specification. If no equivalent product is available we‘ll discuss an alternative settlement with you, and we’ll always do our best to make sure that you’re satisfied with the outcome."
http://www.johnlewis.com/customer-services/information-about-our-terms-and-conditions#cq-second-year
This is in addition to your statutory rights under the Sales of Goods Act that states that if an item develops a fault in the first 6 months you are entitled to a repair or replacement