In the case of a repair, a reasonable time could be classed as one month, but it appears as though there is a difficulty or shortage of parts. On that basis, I would suggest that you contact John Lewis, for further advice. John Lewis would possibly use a third party for repair work, and their methods could be different to John Lewis methods in these circumstances.
How old is the laptop, and was this purchased by credit card.
John Lewis are fairly good regarding resolving these matters, but should there appear to be 'extended' problems, then the credit card company will have 'equal liability' in also resolving the matter under the terms of section 75 of the Consumer Credit Act.
Not quite the same thing except in principle. The sensor on my Pentax Optio 550 packed in (well out of guarantee) but because it was going to take at least three months for the part to be obtained in Germany they offered me a new A20 for their minimum repair cost of £100. Needless to say I accepted!