Best to leave them till late spring/May, if no growth then, i would consider them dead. There are hundreds of other questions over the past few pages regarding Cordylines.

If you've got more patience, wait till end of June - they may by then show new growth either partway up the trunks, or from the base, or both - when that happens, saw down back to where the new growth is. If you have any soggy, oozing, rotting, or orange gunky stuff pouring out, you will need to cut back beyond that point - but don't do it till April if you can wait, it gives the plant more chance of a recovery if you cut it then.

http://www.bbc.co.uk/lancashire/content/articles/2008/01/08/cordylines_feature.shtml

This chap seems to know lots about cordylines you could have a read or even I think ask a question.
Hope you manage to get the advice you require as it would be a shame to lose them

You might have to copy and paste that link it is the BBC Lancashire Nature

Well, Maggy7, I don't know what to think - given I'm a professional and have, in fact given clear advice to Seaportsteve, your suggestion that he consults elsewhere could be taken very much amiss by me, could it not?