jimbob,
You query implies that there must be around 400 or more separate containers within this bund. Are they stacked on each other, on shelving or all sitting on the floor? The reason I ask this question is that the more tins and drums that are on the floor, the smaller the area that can be open fuel surface if a leakage occurs. A portable extinguisher might be capable of putting out such a fire, although the many obstructions are bound to adversely affect its efficiency!
If much is stocked on shelving, a spillage may result in fuel flowing from shelving; this sort of fire can be difficult to control with a portable extinguisher.
When I was dealing regularly with flammable liquids (mostly 205 litre metal drums) we did not bother with an extinguisher to protect the store. We had no ignition sources in the stores, and had strict rules about moving drums about with a proper trolley and decanting liquids only outside the store in the open air. We did keep 25lb (12kg) dry powder extinguishers outside the store but this was primarily for any fire outside the store resulting from a spillage while transferring liquids.
If there is a need to actually fight a fire in the bunded store then you will need either a transportable foam or DP extinguisher - ie one on wheels and with a large capacity compared to portable extinguishers ie 100kg or so of extinguishing agent and someone trained to use this specialised equipment. Alternatives might be a self-contained foam sprinkler system automatically operating on a fire or possibly a water mist system or a hosereel foam unit, although this latter has limitations of capacity.
