Cordylines - I've only grown them in pots as it's easier to keep them tucked up in winter. It's not your usual plant for a rockery, I would suggest putting your young plants in nice pots and trying them in different positions.
The other plants all look like bulbs of various kinds to me. The photos won't enlarge, but in the second one I can see tulips towards the front and daffodils behind.

Agree, they all look like bulbs - with the ones in the first pic between the cordylines, they still look like typical bulb growth, but can't tell for sure. Best not disturbed at this stage, they may abort as they're actively growing and getting ready to flower. I think its not a bad idea to plant a cordyline where the monkey puzzle was, actually, as it grows up it won't block too much light. And you've got a couple of spares in case we have a bad winter later on.

Thanks guys. But is a large pot suitable for a cordyline that I hope to grow taller than my last one at just over 7 ft ?

Hank a cordyline is not a rockery plant... sorry C

Yes Cordyline Australis can grow tall in pots. It will grow faster in the ground. They grow well in wet places like the Welsh and Scottish west coasts where rainfall is over 1000mm per annum. Problems will be keeping it damp and fed and when tall from falling over as it will be top heavy.

Certainly worth potting for a few years. I always stick in a hand full of chicken poo pellets for good green growth.

Thanks very much, I'll think carefully about your replies

My cordyline in a pot was about 7/8ft when it succumbed to probably the same winter weather as yours did. It had flowered and attracted long-tailed tits in winter. Just re-pot as it grows bigger and I didn't do much with it except a feed of chicken poo pellets in spring. Like yours, my dead one has produced baby cordylines and they too are well established now even though I moved the dead plant to a shaded area, to keep the massive plastic pot out of sight really.

The Cordylines did not die - the top growth did. They are root hardy to -20C. Top growth is hardy to around -12C depending on conditions locally and duration. Protecting the crown with lots of fleece in 2010 type weather will help it survive.