Hi Breadlady
Yes, just ensure that the butter you use to bake with is unsalted and not salted. That way, you get to control the overall taste of the bake. You would use the exact amounts as you would for the margarine and interchange them like for like. Just make sure you use a good quality spreadable to ensure you get good results. The better the quality of the ingredient, the better the bake.
Your Cherry Loaf will be dense like the Madeira, and I use Flora Buttery when I make that, so you should be fine. It's always good to make test cakes, though, just to see how your overall cakes turn out. You might find that the overall bake differs slightly from your original ones, but that can be either good or bad, depending on your preferences. Just remember, any time you swap anything out, you may change the texture, taste or even colour, so it's always good to do a test cake just to see how they fare.