These characters are set in England, 1651-1653. King Charles I lost the first English civil war and the second English civil war and then his head, all in the 1640s. In 1651, his son, the future Charles II, attempted to retake the throne - he and his Scots army lost, and Charles retreated to the Continent (Netherlands and France, depending on the year).
In the meantime, most former Royalists accepted the status quo and the new Puritan government, but some did not. Ex-cavalrymen in particular became highwaymen - outlaws with a mission. They had an interesting code of honor, being certain to return enough of a victim's money so they could get home, for example, and were very chivalrous towards the ladies. But they robbed Puritans ruthlessly, and contributed some of their takings to the King-in-exile's war chest.
The genre is a lot of fun - you get to say, "Stand and Deliver!" and there's something satisfying about playing an outlaw who is morally superior to the established government. Not that they really had the moral high ground historically - both sides were pretty morally bankrupt, to be honest. (The delightful history of England entitled 1066 and All That states that the Puritans were Right but Repulsive, while the Royalists were Wrong but Romantic. Um, that's from memory, so consider it simply paraphrased ...)
But you don't tell the players that - just tell them the Royalist side of the story, and they really get into it, and have a blast. (Quite literally, sometimes: there's a demolitions expert in the group, and he started the Great Fire of London 15 years early in one game ...)
As of August 2011, there's a silly series of videos on youtube called Horrible Histories. There's one on Cromwell and the Restoration of Charles II, with links to others on the same era. That's the level of history you tell the players! (And of course there's the classic Oliver Cromwell by Monty Python.)