The Control of Pollution (Oil Storage) (England) Regulations 2001
3. Requirements for storage of oil - general
(1) Oil shall be stored in a container which is of sufficient strength and structural integrity to ensure that it is unlikely to burst or leak in its ordinary use.
(2) The container must be situated within a secondary system which satisfies the following requirements -
(a) subject to paragraph (5), it must have a capacity of not less than 110% of the container’s storage capacity or, if there is more than one container within the system, of not less than 110% of the largest container’s storage capacity or 25% of their aggregate storage capacity, whichever is the greater;