If anyone is interested the HSE say this...
[h=3]"What is the limit of the amount of petrol I can store for domestic use?[/h] The
Petroleum Spirit (Motor Vehicles etc.) Regulations 1929 
and the
Petroleum Spirit (Plastic Containers) Regulations 1982 limit the amount of
petrol that can be kept in a domestic garageor within six metres of a building (e.g. most domestic driveways). The limit is a maximum of two suitable metal containers each of a maximum capacity of ten litres and two plastic containers (which have to be of an approved design) each of a maximum capacity of five litres. These limits also apply to any containers kept in a vehicle parked in the garage or on the driveway (but not to the internal fuel tank of the vehicle). Under no circumstances should the petrol containers be stored in the home itself.
Anyone who wishes to store larger quantities than this, or use larger containers, is required to notify the local Petroleum Licensing Authority (PLA) and to store the petrol in a prescribed manner set out in the 1929 Regulations mentioned above - enquirers who want further details should contact their local PLA. Storage of more than 275 litres (60 gallons) of petrol requires a petrol licence - again, contact the local PLA."
and
[h=3]"How should I store diesel and what quantity am I allowed?[/h] 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"
Extracts taken from
http://www.hse.gov.uk/fireandexplosion/petroleum-faqs.htm#filling
