Straight from the horses mouth -

There are no specific legal requirements on how to store diesel or the quantity allowed either in workplaces or domestic premises. It is not, from a health and safety point of view, a particularly hazardous substance within the meaning of the Dangerous Substances and Explosive Atmospheres Regulations 2002 - its vapour flash point is too high. This means that its vapour will not ignite at normal room temperatures.

That said, there are some general issues you'll need to take into account:

no 'hotwork' should be performed on the vessel unless it is emptied and purged of any remaining vapour.
the drum should be positioned away from any source of direct heat.
the drum should be located in an area where there is no risk of collision with vehicles, fork-lift trucks etc. (diesel splashing onto a hot engine will probably ignite).
leaks and spills should be contained to the vicinity of the drum and mopped up quickly, to lessen the risk of slipping.
refilling and dispensing activities need to take account of manual handling issues etc.
While diesel is not a particularly dangerous substance from a health and safety point of view, it is an environmental hazard, with considerable clean-up costs if it should leak into a drain, watercourse or the soil. You may, therefore, wish to contact the Environment Agency for further information .
http://www.environment-a...ss/topics/oil/32014.aspx

You may find this of use;

http://www.environment-a...siness/OSR_Flowchart.pdf

Also, PPG2 - Above Ground Oil Storage Tanks.

That statement made me smile as it was followed by a link to the correct legislation???????

Yes, focus mainly on environmental legislation such as the oil storage regs (as applicable to England/Scotland etc). If its a farm the rules are slightly different in Scotland and "Google" The Polluter Pays Principle or PPP. Loads of info and different legislation there and reference (already made by others) to the Polution Prevention Guidance or PPG (not powder puff girls) series is valid.

H/S issues? Dont go off and ignor this issue, think COSHH, review rire risk assessment in accordance with the RRO (or Act in Scotland) and even back to basics as implied under the HSWA Section 2.2 (safe handling, storage etc etc).

You may like to consider storing the fuel-oil in a container that will prevent leakage into the environment if the primary storage container is breached.
Any significant leakage into the environment is reportable, the subsequent clean-up will be VERY costly and may involve the removal, and safe disposal, of any contaminated material.