Well, I gave it another go, and it actually does work, abit slowly. Which is probably as the dirt is well and trully embedded in the plastic.
What I do, is I take the cards off the car, put them into the bathtub (shh.. dont tell anyone) and using a dishbrush and dishcleaner shrub the cards a couple of times to get the basic dirt off, let them dry.
I then put them on the kitchen table (shh, especially don't tell anyone that!) and use white spirits to clean the white paint and other stains off, then use ambersil spray on each point that needs doing. Spray on, brush it it harshy into the stain, wipe it off, let it dry. Once its dry, I go after the next large stain, then the next one, and so on until I am reasonably happy that the stains are gone.
I am doing alright now, I think.
When I get to removing dirt from the roofliner and the other parts, then we will see..