have the same problem with some of my cordylines too Hopkirk, sure it was the forst last winter that caused it, I am going to keep feeing fine with just miracle gro!..hopefully it will give the necessary nutrients to keep the plants growing and flourishing!.............We'll see what happens!............
I was thinking lawn feed, as that greens up the grass quite quickly, but then I guess that is mostly new growth.
I presume I won't get new green foliage until it grows new leaves, and thankfully there are signs of that happening.
I have bought a general purpose plant food, and will try that.