It's good news that the dealer has agreed to replace the battery under warranty, as this would be an expensive replacement if you were paying for it yourself.

It's difficult to assess what this would add in value as far as a trade-in goes, but given the cost of the work could easily be in excess of £3000 we would expect at least some of this to be reflected in the trade-in value. If you want to get the maximum value for this repair we would suggest selling privately, as including the fact that the car has a new drive battery would be very attractive to anyone looking to purchase a used Mitsubishi Outlander PHEV.