Clear any leaves from the tree which are trapped in the cordylines before wrapping for the winter - usual procedure to protect the growing tip is to pull all the leaves up to the top, like a ponytail, then secure them in that position with string. Then wrap horticultural fleece around that and the trunk, bubblewrap the pot to insulate the roots. To be done at start of winter, removed when weather warms up in spring. Best if you can take the fleece off during milder spells over winter though, and replace as necessary.

I did the same as Bamboo suggested but instead of fleece I cut a pair of tights in half and pulled the leg over the ponytail, it kept it together and stopped leaves getting inbetween cordylines leaves.