AMP Insurance

We have had our car a year and a half we bought it second hand from a dealer A&J motors Richards Bay. We noticed oil in the water bottle so took it to Dups auto in Parys. There they checked the radiator and oil cooler that was all good. So next step was to remove the head to check for a blown gasket once the head was removed they found that the one sleeve had a crack in it. So we contacted our extra insurance company AMP Insurance as the quote is standing at R48000. Amp then sent a assessor out he took photos. They then sent a sms to us stating Dear policyholder we are unable to process your claim due to improper repair to the vehicle or parts/materials supp**** for repair Regards AMP. Our car has never been repaired by any one while it has been with us. We have been left with no car since this hole process has begun. When we contacted AMP today they said the reason beginning that there is pratley steel on the underside of the water pump it is not leaking it is on the underside so we could not see it. Because of this done prior to us purchasing the vehicle they say we are liable. How can we be. Please help