This heart felt plea had me searching the archives to enact a cure.... Hmm! Let me see.
First off, are you sure of the source of the noxious staining?
The "alleged urination" in question might have been enacted by a couple of imbibers that had repeatedly lost moisture for a couple of days.Their liquid intake had weakened the uric acid content in their bodies to the extent that even in their own home site there was no trace of the "Spot" that they had used, so I am loath to blame them for discoloured red paint on the Porky, however...
However I am informed by the local bodywork witch-doctor that you will need fourteen drops of blood drawn at midnight from the left wrist veins of two willing donors who must drive 1; a BMW Minicooper and 2; A Nissan Skyline, whatever that is. Apparently this has been tried with an EVO but harmful results resulted (I dunno, I asked on your behalf, OK)
This needs to be added to two gallons of finest London water, distilled during the hours before midnight and the offending paintwork should be washed with it before dawn. Fortunately no further ingredients are required.
A small amount of wax based detergent is allowed but not encouraged....
Oh yes, and never get mechanics upset with your behaviour, it seems they have ways of getting you later.