In consideration of payment of additional premium the cover is extended to cover loss or damage by spoilage resulting from the retardation or interruption or cessation of any process or operation caused by any of the perils covered under the policy provided that liability for destruction of or damage to the property insured is first admitted by the company. The Add on cover may be for all stocks and machinery, containers and equipment in specified blocks, specified sums being declared for each block and must be made subject to Average.