Thanks for the continued input all.
I picked up the Lion Rampant rules fairly cheaply so I'm just having a read of those right now. Looks straightforward, but scope for depth and tactics. I like the idea of issuing a leader-on-leader challenge, and there are a few cool scenarios to try. I can't see why this would not work in 15mm.
In terms of a time period, I like Dark Ages but having chatted with a friend, late-Medieval throws in a bit more variety with some early artillery. Perhaps 100 Years War, War of the Roses, or even Crusades. No idea which army/retinue to go for, I need to do some research basically!