I have a 12ft cordyline with 5 branches groing from the top

If you have no new growth at the top of the tree, or the trunk is rotting or smells bad, cut it down to the base, leaving the new growths at the bottom to take over. If you have any signs of new leaves at the top, just remove the dead and dying leaves. The growth at the base will grow on, so the plant isn't actually dead, just might have been killed right back to its roots, which is why the little plantlets are appearing there.

Thank you so much for your reply. I think the top of the tree is dead and I will therefore take your advice and cut it down. However, when you say to cut it down to the base should i actually take it down to ground level or shall i leave a small amount of the trunk as i dont want to damage the 2 small shoots which have grown to approx. 10 inches in height in the last couple of weeks.Also, is there any specific plant food i can buy to encourage healthy growth. Thank you.

They're not particularly hungry plants in the ground, so I wouldn't worry about feeding. Certainly cut the trunk down to a point where you're not going to cause damage to the new parts, and if that means a bit of a stump, well that's okay - if those babies grow on, you won't see it anyway eventually.

Thanks for that.