In all seriousness, try Flash Powder if you can find it or watered down cream (Jif/Cif would be worth a try if you can't find Flash) , in a strong solution, it is slightly alkaline and we used to use it to clean engine casings from built up dirt prior to repainting.
Also, Fenwicks
http://www.fenwicks.info/products page.htm make a good range of mostly caravan cleaners that are quite effective and have a much wider range of uses - the caravan cleaner was used to clean filthy mountain bikes by many before they introduced a specific cleaner.
Another option is TFR, Traffic Film Remover, or truck wash, this is a fairly harsh alkaline (caustic) cleaner for heavy road dirt.
I would recommend using gloves and eye protection with any alkaline/caustic cleaners, for your own good. If the bunkers are the ones I think they are, good luck, last time I saw them they were pretty caked.
Still though, I suspect that a large dolop of elbow grease and a scrubbing brush will be required.