It's very rare for problems to occur with the 1.2 PureTech. If they won't automatically fit a new engine then write a letter to the dealer principal of the supplying dealership stating that you find it unacceptable that your engine should have had these problems as a result of a broken spark plug, that you are very concerned that there has been "consequential damage" and that you hold the dealership fully responsible for any engine problems you suffer for up to six years from the date of purchase. Send this letter by Post Office Special Delivery, keep a copy, and staple the certificate of posting to the copy so it becomes a 'matter of record' admissible in court.