I agree about widening the rear notch -- it helps with sight acquisition and eye-strain and doesn't hurt accuracy much if at all in my experience, since more light on either side of the blade will aid in centering it. I wanted "fine" sights on my flintlock, but have settled for low with a wide notch, which looks more correct from any reasonable distance but is easier to use. Easy to try -- just take a needle file (I like square with a front blade) and cut it out. I think Daryl also showed an express-like sight (bead front and wide v in back) he had made -- if it fits the rifle, it is one of the best types for ease of use. I would think it would fit the look of an English sporting rifle very well.
Additionally, neither the rear sight nor the target really need to be in sharp focus, as long as the front sight is clear. Sounds counter-intuitive, but it works.