My 2500 litre single-skin plastic tank was installed by an Oftec engineer 12 years ago on a 5 inch thick concrete base, 2 feet longer and wider than the tank. On 9th March, following a refill, a 2 inch split appeared half way up the tank. I plugged it with a piece of soap, dried the area and applied black silage-bag tape. So far all is well, but I am planning to install a replacement bunded tank soon. This happening indicates that there are stresses in a plastic tank, which after seasonal heating and cooling, plus emptying and filling, can suddenly cause a split to appear. So it is better to buy a bunded tank at the start.