I need to update my oil storage tank base. What should I do?

To comply with British Standards, the base material should be either:

Concrete at least 100mm thick.
Paving stones at least 50mm thick.
Stonework at least 50mm thick.

The overall size of the base should be larger than the oil storage tank and any integral oil storage tank bund, so that when the oil storage tank is installed, the base has a clear projection of a minimum of 300mm around all sides of the oil storage tank.