First are you sure that it is creosote? Stuff has been banned since 1998 by the EU (bless 'em) for all except certain professional users. Its tough to shift if it is so it would be as well to know if it definately is.
If it is try oil patch remover and /or a good blast with a pressure washer.
You're right - it's creosote substitute not creosote. White spirit helped - with a lot of scrubbing. Will need a lot more work.
By the way, armillatox is not a pesticide, it's a disinfectant/cleaner. A bit like Jeyes Fluid only more so. It can be used to disinfect soil from e.g. rose black spot. It is also recommended as a cleaner for patios, paths, etc.
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