Many thanks again chaps, it is appreciated.
As the dirt, staining and weathering has been 'laid down' on a gloss coat, there is still much to do, as the matt clear coat will alter the appearance, slightly diffusing the overall effect.
With the matt coat on, more surface wear and tear, and certainly more underside grime will be added, as this will be more realistic 'on the surface', rather than being diluted by clear coats.
That said, I don't want to make it look like a "nicely weathered model", with over-accentuated panel lines etc., and hope that I can get a subtle, worn and dusty look, including rain streaks and other stains in the covering dust.
However, as much as I'd like to make it look as "it was", with the filth, dirt, dust etc at the time, as a 'stand alone' model, on the shelf. without benefit of a diorama base, I don't want to overdo things and make it look too ..... er .... Hollywood ?