I spent quite a bit of my spare time thinking about the Portable Wargames Rules for the C18 and came up with what I thought was quite a nice set.
However when I read through them again they seemed quite familiar. Yep I had almost managed to rewrite Twilight of the Sun King. We had a look at these a while ago using the free set available on Steve's Balagan (but nothing came of it and we stuck with Volley and Bayonet). They must have remained in my subconscious.
The free set are still here:
There is however a more recent iteration available from The Pike and Shot Society. I ordered these for the princely sum of £3.45 (inc p&p). They arrived the next day - excellent service.
At this price you don't get a Black Powder type book, more like the basic rule books of old. They are a grand tactical set of rules based on the brigade - 1cm =100 paces with combat resolution based on a simple morale test. With a couple of tweaks for the grid system they should make a quick game.
And, if you like that sort of thing, there is an active Yahoo group.