The best time, I find to weather wood is before sealing it. Mind you if you get it wrong when the wood is open grained, you are stuffed. As your decks are already sealed, I would use washes of dark grey and pale gray to represent dirt and sun bleaching respectively. Preferably oil based paints such as the Humbrol range, thinned with either white spirit or proper thinners. Apply small amounts of paint along plank edges, wherever dirt would accumulate. This can be washed out with the brush you are applying it with or dabbed off with a bit of kitchen cloth.
Another effect you could try is a bit of dry brushing. Your Matt Cote finish seems to still have a bit of a sheen to it, you could try thinning the Matt Cote up to 50% with thinners, stir it up for about five minutes, it is prone to seperate the matting agent when left for any length of time. Dip your brush in, dry it almost completely on a bit of kitchen cloth or newspaper, then draw the brush up and down the planking where 'wear and tear' would be most likely, laying down a small amount of , hopefully totally flat varnish. this will give the effect of a worn out surface. you can also represent this with wire wool of fine wet and dry paper, however this is a bit brutal...