Another issue has cropped up recently - what do you do with the resultant water contaminated with whatever you wash out of the engine?
The Environment Agency probably won't grant you discharge consent - if you let it run down a drain or onto the ground you could be prosecuted for a pollution offence.
The only legal method of carrying out this procedure that we can see at the moment is to catch ALL of the water coming out of the engine and have it taken away by a licenced waste contractor and disposed of properly.
Incidentally, during aircraft washing procedures at LGW, where we have special interceptors to prevent the runoff getting into the groundwater and local water courses, we've found significant traces of cadmium and magnesium. A/c manufacturers tell us this must mostly have come from undercarriage components. All this contamination has to be disposed of at considerable cost.
Cheers,
TheOddOne