Any person who suffers a loss through your negligence can demand compensation for such loss and pursue the matter via the County Court if necessary. (It would be for the court to decide what expenses could be justified but they could certainly include the costs of wasted travel, etc).
If Barmaid (who is a barrister in the civil courts) posts here her advice should definitely take precedence over mine but my own advice would be for you to make a written offer of compensation (containing the words "without prejudice"), for an amount lower than that which is being claimed, and hope that (in order to to get a quick settlement) your customer accepts it.
Chris