B&Q have a road traffic film remover they sell alongside their pressure washer range. Seems pretty good...... Any good autofactor will also have degreaser that will remove most dirt.... use it neat on a rag & wash off really well with warm water and washing up liquid.... A good coat of polish then...
I use a product called "gunk"... this is used in the auto industry to clean engines... put on with a rag and washed off with water... it is water soluble.
A coat of polish helps stop the build up of dirt again.......... then again my AEROplane is composite.
Jon