"Is it legal?"
Is what legal? John Lewis has an impeccable reputation for customer service, and the company is unlikely to invent the fact that the power socket is damaged if it isn't - why would they want to do that?
These things always end up being one person's word against another's, but if JL is adamant there is little that you can do. That said, £900 for a power socket repair does seem a lot. The damage would not be covered by the warranty, as it would be classed as a user-caused problem.
You can try arguing the case with JL, but I wouldn't hold out too much hope.